Magnesium Cyclopentadienyls

Magnesium Cyclopentadienyls

Mg has been found to be optimal p-dopant for GaN despite its intrinsic difficulties. During the growth process of Mg doped GaN, atomic H generated upon decomposition of NH3 is forming  Mg-H complexes in the layer. This has been demonstrated by the occurrence of LO mode in IR absorption, and by the observation of the Mg-H local vibration modes. This H passivation limits the electrical activity of Mg, therefore an activation process is required to get full activation of the Mg atoms.

Among other metalorganic Mg MOCVD precursors, magnesium cyclopentadienyls are the most common precursors used for the p-doping of GaN based materials.

Use of Mg(C5H5)2 and O2 as oxygen source allowed to prepare epitaxial MgO films by MOCVD. [4]

Bis(cyclopentadienyl)magnesium MgCp2 (Magnesocene)

Fig.: Structure of magnesocene in solid (left) and in the gasphase (right).

Bis(cyclopentadienyl)magnesium MgCp2

Bis(cyclopentadienyl)magnesium MgCp2 is a air- and moisture-sensitive solid with a moderate vapor pressure. It is the most popular in industry p-doping precursor for GaN materials, however ts solid nature is causing issues in long term with evaporation stability (crystallite caking, channel formation in bubblers etc.)

The first synthesis of bis(cyclopentadienyl)magnesium was reported by E.O. Fischer und G. Wilkinson in 1954. [119, 119] Gasphase electron diffraction studies revealed ecliptic (rotated by 0°) conformation of the parallel Cp-rings (Fig. right) [] However, single crystal studies showed that in solid state C5H5-rings are rotated by 180° (Fig. left)   []

Magnesium bis (cyclopentadienyl) MgCp2 for MOCVD/MOVPE applications

MgCp2 forMg metal growth by CVI

MgCp2 was successfully applied as precursor for chemical vapor infiltration (CVI) of magnesium metal film on carbon foams at 700-775°C substrate temperature. Optimal growth pressure was 600 torr (only negligible deposits were obtained at pressures lower than 300 Torr), precursor efficiency was 19-25%. Best results were obtained without H2 process gas: dense, adherent Mg coatings deposited. With H2 (used for hydrogen reduction of MgCp2) poorly adherent layers obtained, probably due to hydriding during deposion), no advantage in terms of oxygen content was observed. [116]

MgCp2 forMgO growth by MOCVD

MgCp2 has been applied as precursor for the MOCVD growth of MgO as passivating buffer layers for YBa2Cu3O7−x deposition on GaAs substrates. MgO buffer layers at relatively low substrate temperature could be deposited [123]

MgCp2 forMgO growth by ALD

Bis(cyclopentadienyl)magnesium MgCp2 has been applied for the growth of magnesium oxide thin films by atomic layer epitaxy (ALE) with water as oxygen source, on soda lime glass and Si(100) as substrates. Growth was studied in the temperature range between 100 and 400 °C;  surface-controlled growth was observed at 200–300 °C on both substrates.  Growth rate of 1.16 Å/cycle is almost an order of magnitude higher than the ALE growth rate obtained with β-diketonate-type precursors and ozone or hydrogen peroxide. Films grown over 200°C were polycrystalline with (111) dominant orientation on both substrates; films grown below 200°C were amorphous. Films were stoichiometric at growth temperatures 200–400 °C according to XPS and TOF-ERDA; with contaminant levels were 0.5 at% H and 0.1 at% C (300 °C). Roughness of the layers was deposition temperature- dependent: films with rms values of 8–10 nm were grown at 250–350 °C, while above or below this temperature range rms  2–4 nm was observed.  High hydrogen content has been observed in the low temperature MgO;  it was attributed to the increase of the ---OH groups in the film. [124]

MgCp2 was used for MgO thin films growth on Si using ALD at substrate temperatures 500-900°C. XRD spectra of MgO show that orientation changes from [111] to [100] when increasing substrate temperature from 700 to 800°C. Crystallite grain size decreases at 500-600°C and then increases at 600-900°C temperatures according to SEM [126]

MgCp2 for MgAlO growth by ALD

MgCp2 in combination with (CH3)3Al and H2O were used as precursors for the self-limiting ALD growth of MgAlO thin films at 100–400 °C onto soda lime glass and Si(100) substrates. Uniform and stoichiometric MgAl2O4 films could be deposited by adjusting the pulsing ratio of metal precursors in the range Mg:Al 5:1 to 1:9. Films deposited at 100 °C contained high impurity levels (19 at.% H and 2.4 at.% C), that decreased to 0.7 at.% H and <0.1 at.% C when the deposition temperature was increased to 300-400 °C. Oxygen content was increasing from 47 to 56 at.% when deposition temperature was increased from 100 to 200 °C and stabilized at 56 at.% at higher temperatures ( stoichiometric value 57 at.%). As-deposited films were amorphous but crystalline MgAl2O4 with preferable (111) spinel texture was obtained when films on Si(100)  deposied at 300°C were annealed at 800–1000 °C/10 minutes in a N2 atmosphere. (Fig , XRD and AFM, 130nm MgAl2O4 film)

Deposition rate obtained with the Mg to Al precursor pulsing ratio of 1:2 was approximately 90% from the theoretical growth rate calculated from the binary oxide depositions.

According to the AFM measurements, roughness of the as-deposited films on Si(100) was significantly dependent on the pulsing ratio of the precursors, with rms increasing from 0.4 to 10 nm with increasing relative MgCp2 pulses (Fig, film thickness ~150nm, Inset shows AFM images of the Mg–Al–O films deposited with the MgCp2/H2O to AlMe3/H2O pulsing ratio of 1:2 (left) and 2:1 (right)) [125]

MgAl2O4 films have been attempted to deposit by an ALD-related process using MgCp2, AlEt3 and H2O as precursors at 370–900 °C. Deposition was successful in the temperature range of 600–750°C, limited by the low reactivity and decomposition of AlEt3. Film growth rate / verification of the self-limiting ALD deposition mechanism were however not reported. [127]

MgCp2 as a p-dopant in III-V materials

Bis(cyclopentadienyl)magnesium MgCp2 is the most commonly used p-dopant in GaN-based materials as well as GaAs, AlGaAs, InP. [[i],[ii]]

MgCp2 has been used for the p-doping of GaAs. The dependence of doping level (hole concentration) of GaAs using MgCp2, with and without AlGaAs buffer, on dopant mole fraction and growth temperature are displayed in Fig. [[iii]] Doping efficiency is dependent on the presence of oxygen/moisture in the reactor and on the reactor walls: use of AlGaAs buffer layer getters water and oxygen from the reactor walls, improving the incorporation efficiency. Mg incorporation with an AlGaAs buffer follows a square law dependence on the MgCp2 mole fraction. Mg incorporation drops rapidly with increasing growth temperature due to rapid evaporation of the dopant from the growth front.(Fig.)

MgCp2 for Mg-doped GaN layers

MgCp2 was used as precursor for the Mg doping of GaN layers grown by MOCVD on AIX 2000 HT Planetary® Reactor carrying 7x2” sapphire substrates. Mg doping concentrations of over 1·1020 cm-3 resulted in hole concentrations up to 1·1018 cm-3. p-type carrier density uniformity around 10 % (st.dev.) was observed revealing incorporation uniformity as well as influences of the activation process.[131]

GaN layers doped by MgCp2 (vaporized either as a solid or as a “Solution Cp2Mg”™), were studied by wavelength dispersive x-ray spectrometry (WDX-EPMA) as well as SIMS. No systematic differences were seen between the solid magnesocene and Solution Cp2Mg™. A ~40% reduction in Mg incorporation was observed with reduction of growth temperature from 1130 to 1090°C. No systematic structural degradation of GaN:Mg close to the magnesium solubility limit (~1020 cm−3) was observed. Up to a half of the saturation limit, [Mg] values were proportional to the magnesium source flow, and indicated magnesium atom incorporation from the gas phase with ~11% of the efficiency of gallium atoms. The most conductive sample had a hole concentration of 4.4 × 1017 cm−3, consistent with the expected generation of acceptors from only a small fraction of the magnesium atoms. [132]

Mg-doped GaN based laser diodes were grown by MOCVD using MgCp2 as a dopant. Uniformly distributed precipitates were observed by TEM in the p-type layers of laser structures; the precipitate density decreased with decreasing flow of MgCp2, which affects the hole concentrations in the p-type layers. Higher hole concentration combined with reduced precipitate density, reduced the threshold current density by 30% (from 20.8 V to 14.3 V) and improved the internal quantum efficiency due to the higher number of holes available for radiative recombination. [133]

MgCp2 was used as a dopant for the growth of Mg-doped GaN. Hole concentration was dependent on the way of dopant and reactant supply, substrate and buffer layer:  6x1018 cm-3 on sapphire by alternate pulse MOVPE technique (alternate Ga and Mg pulses in the constant NH3 flow), 2x1018 cm-3 (constant feeding of Ga and Mg sources); 2x1019cm-3 on a low temperature AlN buffer layer by using alternate feedings of Ga source and NH3 with Mg-Si co-doping, 6x1018 cm-3 on an AlGaN buffer on a SiC substrate by alternate co-doping technique. The activation energies for Mg-doped GaN grown by the pulse feedings of source materials were lower than that with continuous supply of sources.[134]

According to DLTS (deep level transient spectroscopy), deep levels measured at 0.26 and 0.62 eV below the conduction band were found in relatively low concentrations of 2x1013 cm-3 in undoped GaN Addition of small quantities of the Mg acceptor species using MgCp2 to the n-type GaN films during MOCVD growth corresponded to a significant increase in the concentration of the level at 0.62 eV. The concentration of the shallower level, found to be independent of the Cp2Mg addition, remained unchanged.[135]

When underneath p-type GaN layer was grown by MOCVD for the nitride-based LEDs with Si-doped n+-In0.23Ga0.77N/GaN short-period superlattice (SPS) tunneling contact top layer, it was found that the LED-operation voltage was almost independent of the p-dopant CP2Mg flow rate. [136]

Defects formed in the MOCVD-grown GaN doped using MgCp2 (either delta-doped or continuously doped), were studied by TEM. In the samples with Mg delta doping spontaneous ordering was observed consisting of Mg rich planar defects (showing characteristics of inversion domains) on basal planes separated by 10.4 nm and occurring only for growth in the N to Ga polar direction (000 N polarity). On the opposite site of the crystal (Ga to N polar direction), where the growth rate is an order of magnitude faster compared to the growth with N-polarity three-dimensional defects were formed: pyramidal and rectangular, empty inside with Mg segregation on internal surfaces. These types of defects were not observed in the layers where Mg was added continuously.[137]

The diffusion characteristics of Mg dopant in MOVPE-grown GaN grown on Al2O3 with Cp2Mg, TMGa and NH3 precursors, were studied.. At 1000°C – 1300°C growth temperatures the back diffusion of magnesium was observed. Abrupt diffusion profile was observed by SIMS when Cp2Mg partial pressure was increased. Applying Boltzman-Matano analysis of dopant profile it was shown that diffusion coefficient was increasing larger than one order of magnitude within the concentration range: 5x1017 - 1x1020 cm-3. Increasing growth temperature of Mg doped GaN caused shift of diffusion profiles towards layer-substrate interface.[138]

GaN-based LEDs were grown on c-plane sapphire by Low Pressure MOVPE on AIXTRON AIX200 reactor) using Cp2Mg as p-dopant for pGaN layers. Visible light emission was obserced from GaN pn-homojunctions and InGaN heterojunctions. [139]

H-atom incorporation in Mg-doped GaN grown using Cp2Mg by MOCVD was found to increase linearly with Mg concentration, suggesting the formation of simple complex between Mg and H atoms in GaN. Decrease of H-atom concentration was observed after thermal treatment in Ar, supporting the hypothesis that H-atom extraction plays an important role in obtaining low-resistivity p-type conduction [140]

MgCp2 was used as the p-dopant source for the growth of p-type GaN films on sapphire substrates in a horizontal MOCVD system. The acceptor concentration in the post-growth annealed GaN samples increased with the Mg flow rate and reached a peak value of 1x1019 cm-3 at Mg flow rate of 0.84 μmol/min. The films remained semi-insulating even after annealing when the Mg flow rate is higher than 1.08 μmol/min. Postgrowth annealing at 800°C, 30 minutes in N2 ambient is sufficient to activate most of the Mg atoms; p-type acceptor concentration obtained using rapid thermal annealing of Mg-doped GaN is comparable to the furnace annealing process. GaN LEDs using undoped layer as the n-type base layer in a p-on-n structure had light emission spectra dominated by the 430 nm peak, accompanied with two relatively weak peaks located at 380nm and 550nm. [141]

p-type Mg-doped GaN Samples with various doping concentrations were prepared by MOCVD using MgCp2 as Mg source. Variable-temperature Hall effect measurements revealed that as the dopant density was increased to the high values typically used in device applications,  the effective acceptor energy depth decreases from 190 to 112 meV, impurity conduction at low temperature becomes more prominent, the compensation ratio increases, and the valence band mobility drops sharply. The measured doping efficiency drops with Mg concentration above 2x1020 cm-3. [142]

The influence of  MgCp2 flow rate and pGaN growth temperature on electrical and optical properties of InGaN/GaN MQW LED structures was investigated. As the Cp2Mg flow rate increased at constant growth temperature, an operation voltage decreased from 5.18 to 4.34 V at 20 mA injection current. Light output power was measured as a function of injection current at room temperature; highest value was achieved at Cp2Mg flow rate of 1.06 μmol/min. LED operation voltage at RT decreased with increasing p-GaN layer growth temperature. The highest light output power measured as a function of injection current at room temperature was highest at growth temperature of 1050℃. Optimum output power-current-voltage characteristics were achieved with MgCp2 flow rate 2.13 μmol/min and 1050 ℃growth temperature. Light output power is more sensitive to growth temperature than Cp2Mg flow rates in a certain growth condition range of p-GaN:Mg layer.[143]

Low temperature photoluminescence of MgCp2-doped GaN has been investigated.  The Blue band (BB) peak of as-grown samples with higher Mg concentration is observed at lower energies, but it shifts after annealing in N2, stabilising at 2.92 eV at 850°C anneal temperature. This results has been explained by a model based on compensation eefect. [144]

 “Yellow luminescence” in PL spectra of GaN, including MgCp2-doped pGaN, was investigated. The difference between n-type and p-type doped GaN has been discussed [145]

MgCp2 as dopant for InGaN layers

MgCp2  has been applied as source for Mg doping of MOVPE InGaN (In content 0.05 to 0.37). SIMS analysis revealed that Cp2Mg memory effect as a doping source deteriorates the controllability of Mg doping level and profile, especially for thin (-0.4 μm) InGaN layers. Higher Cp2Mg flow rate was needed to get p-type conduction in InxGa1-xN with higher In content: Cp2Mg/(TEG+TMI)≈0.5% for x=0 (GaN), ≈2% for x=0.05 and ≈4% for x=0.2 (p-type conduction was obtained for In content up to 0.2). Such high Cp2Mg flow rate  was needed due to the high residual donor concentration (1019-1020 cm-3) of InGaN films and the low activation efficiency of Mg. The crystalline quality of InGaN was deteriorated with increasing In content as well as Mg doping level. To achieve a p-type InGaN with a lower Mg doping, it was essential to improve the crystalline quality of non-doped InGaN, what  was achieved by the use of a thicker GaN interlayer.[146]

MgCp2 was applied for the p-type doping of MOVPE-grown InxGa1-xN (x∼0.4) Mg-doped InGaN films with up to 23% In show p-type doping with a net acceptor concentration ~5x1016 cm−3. The sample with 37% In showed phase separation when MgCp2 flow rate is increased. By using In0.23Ga0.77N films, an n+-p homo-junctions were fabricated on a GaN template; the response to the AM1.5 illumination was observed and Voc =1.5 V and Jsc=0.5 mA/cm2 were obtained.[147]

Cp2Mg was used for the growth of Mg-doped p-InGaN layers with ~10% In by MOCVD) . Hole conc. in p-InGaN increases with increasing annealing temperature in the range 850°C. Hole mobility remains nearly unchanged, but at too high annealing temperature decreases. Using conductive p-type InGaN layers, p-In0.1Ga0.9N/i-In0.1Ga0.9N/  GaN junction structure was grown and applied for photodiodes preparation. The spectral responsivity of the InGaN/ p-i-n photodiodes shows that the peak responsivity at zero bias is in the wavelength range 350–400 nm. [148]

MgCp2 was used as p-dopant for MOCVD growth of InGaN/GaN five-period multiple quantum wells (MQWs) with different magnesium doping levels.  Magnesium doping led to a smoother surface morphology by AFM. Increase of magnesium doping concentration resulted in the decrease of V-defect density from ∼109 cm−2 (no doping) to ∼106 cm−2 (Cp2Mg: 0.04 sccm) and further to 0 (Cp2 Mg: 0.2 sccm). PL measurements indicated that magnesium doping resulted in stronger emission, which can be attributed to the screening of the polarization-induced band bending. Magnesium doping had no effect on the indium composition and MQW growth rate according to XRD. These results suggest that magnesium doping in MQWs might improve the optical properties of GaN photonic devices.[149]

Cp2Mg was applied for the growth of  Mg-doped In-rich InGaN and InN by MOCVD. Growth temp. was 550°C (1st sert), or increasing temperature with Ga content (2nd set). Upon annealing, p- InGaN was obtained from 2nd set with In content >50%, with an acceptor concentration of 1·1019 cm-3 and mobility of 1– 2 cm2/Vs. None of the samples grown at 550°C showed a p-behavior after heat treatment. [150]

MgCp2 as dopant for InN layers

Bis(cyclopentadienyl)magnesium MgCp2 has been used as a Mg source for the growth of p-doped InN layers by MOVPE. Fine grained InN(Mg) films were obtained; however those  grown at the upstream end of the 18 cm long susceptor contain a high level of C and H contamination (XPS) due to formation of adducts of MgCp2 and NH3. Adduct formation is suppressed by using optimum susceptor position from the upstream end.[151]

Cp2Mg has been tested as a Mg source for the Mg doping of InN during atmospheric-pressure MOVPE. Despite Mg content being proportional to Cp2Mg/TMI molar ratio, all layers were n-type with electron concentration increasing with increasing CP2Mg/TMI ratio. SIMS revealed that C and H contamination was reduced by optimising substrate position on the susceptor. The AFM studies showed that the grain growth of InN was suppressed by the Cp2Mg supply.[152]

MgCp2 as dopant for AlInN layers

MgCp2 was used as a p-dopant in AlInN layers grown by MOVPE (20.9% In and 219.6 arcsec FWHM by HRXRD,  sheet hole concentration of 4.73×1012 cm–2 after thermal annealing). Higher Cp2Mg flow rate decreases the conductivity of AlInN layer due to increase of Al content, but increases the surface roughness, suggesting that lower Cp2Mg flow should be used for preparation of p-type AlInN cladding layer nearly lattice-matched to GaN for optical device applications.[153]

MgCp2 as dopant for AlGaN layers

MgCp2 has been applied for the high temperature (1050°C) MOVPE growth of Mg-doped AlGaN electron-blocking barrier, used for the preparation of the blue InGaN/GaN multiple-quantum-well light-emitting diodes on sapphire substrates. The increase of Cp2Mg flow rate (50 to 200 sccm) resulted in >90 % luminescence efficiency improvement. The structures were characterised by I-V, C-V and electroluminescence measurements. SIMS revealed the Mg doping profiles close to the quantum-well active region. The increased Mg concentration resulted in improved hole injection and carrier concentration, in agreement with the increased electroluminescence efficiency and luminous intensity and with the lower turn-on voltage. [154]

High-performance Mg-doped p-type AlxGa1–xN (x = 0.35) layers were grown by LP MOCVD on AlN/sapphire template. It was found that the p-type resistivity of the AlGaN alloy demonstrates a marked dependence on the Mg concentration, V/III ratio and group III element flow rate. A minimum p-type resistivity of 3.5 Ω cm for AlxGa1–xN (x = 0.35) epilayers was achieved. [155]

Cp2Mg was used as source for the preparation of Mg-doped Al0.1Ga0.9N/undoped GaN heterostructures by MOCVD.  Two distinct hole traps were observed by deep level transient spectroscopy (DLTS): AD1 with an activation energy of 0.18 eV associated with defects in the Al0.1Ga0.9N/GaN interface, and AD2 with an activation energy of 0.88 eV strongly related to Mg-related deep levels, which contribute to the strong blue emission observed by photoluminescence measurements. The DLTS peak intensity of the AD2 trap steadily decreased with decreasing the Cp2Mg molar flow rate [156]

MgCp2 as dopant for GaAs and AlGaAs layers

Early studies reported unsuccessful use of Cp2Mg for the growth of p-doped GaAs and AlGaAs having abrupt transitions by atmospheric pressure MOVPE, as long doping tails were observed. [128]

However, later reports proved that Cp2Mg could be used as Mg p-dopant precursor for the preparation of extremely abrupt p+-n doping transitions in GaAs and AlGaAs layers grown by MOVPE. Mg incorporation was found to depend linearly on the Cp2Mg concentration, in contrast to reports of a supralinear behaviour. Methods for eliminating reactor memory effects were developed. Layers doped to ~1 x 1019 cm-3 and as thin as 100 nm at 10% of the peak doping have been grown under conditions compatible with growth of high quality Al0.3Ga0.7As.[129]

High-quality Mg-doped p-GaAs layers (p on n GaAs solar cell with an efficiency of 24% at air mass 1.5 (AM1.5)) were grown by vacuum MOCVD by using  MgCp2 purified by multiple vacuum sublimations to reduce the contamination by air and by cyclopentadiene (CP) by an order of magnitude. [130]

MgCp2 as dopant for AlGaInP layers

Cp2Mg was tested as p-doping precursor for MOCVD-grown Al0.5In0.5P used as cladding layers for 630nm band AlGaInP laser diodes, dopant concentration profile were studied. Problems of small band discontinuities and proper p-doping in terms of achieving high barriers for a reduction of thermal carrier leakage into the

MgCp2 forMg metal growth by CVI

MgCp2 was successfully applied as precursor for chemical vapor infiltration (CVI) of magnesium metal film on carbon foams at 700-775°C substrate temperature. Optimal growth pressure was 600 torr (only negligible deposits were obtained at pressures lower than 300 Torr), precursor efficiency was 19-25%. Best results were obtained without H2 process gas: dense, adherent Mg coatings deposited. With H2 (used for hydrogen reduction of MgCp2) poorly adherent layers obtained, probably due to hydriding during deposion), no advantage in terms of oxygen content was observed. [116]

MgCp2 forMgO growth by MOCVD

MgCp2 has been applied as precursor for the MOCVD growth of MgO as passivating buffer layers for YBa2Cu3O7−x deposition on GaAs substrates. MgO buffer layers at relatively low substrate temperature could be deposited [123]

MgCp2 forMgO growth by ALD

Bis(cyclopentadienyl)magnesium MgCp2 has been applied for the growth of magnesium oxide thin films by atomic layer epitaxy (ALE) with water as oxygen source, on soda lime glass and Si(100) as substrates. Growth was studied in the temperature range between 100 and 400 °C;  surface-controlled growth was observed at 200–300 °C on both substrates.  Growth rate of 1.16 Å/cycle is almost an order of magnitude higher than the ALE growth rate obtained with β-diketonate-type precursors and ozone or hydrogen peroxide. Films grown over 200°C were polycrystalline with (111) dominant orientation on both substrates; films grown below 200°C were amorphous. Films were stoichiometric at growth temperatures 200–400 °C according to XPS and TOF-ERDA; with contaminant levels were 0.5 at% H and 0.1 at% C (300 °C). Roughness of the layers was deposition temperature- dependent: films with rms values of 8–10 nm were grown at 250–350 °C, while above or below this temperature range rms  2–4 nm was observed.  High hydrogen content has been observed in the low temperature MgO;  it was attributed to the increase of the ---OH groups in the film. [124]

MgCp2 was used for MgO thin films growth on Si using ALD at substrate temperatures 500-900°C. XRD spectra of MgO show that orientation changes from [111] to [100] when increasing substrate temperature from 700 to 800°C. Crystallite grain size decreases at 500-600°C and then increases at 600-900°C temperatures according to SEM [126]

MgCp2 for MgAlO growth by ALD

MgCp2 in combination with (CH3)3Al and H2O were used as precursors for the self-limiting ALD growth of MgAlO thin films at 100–400 °C onto soda lime glass and Si(100) substrates. Uniform and stoichiometric MgAl2O4 films could be deposited by adjusting the pulsing ratio of metal precursors in the range Mg:Al 5:1 to 1:9. Films deposited at 100 °C contained high impurity levels (19 at.% H and 2.4 at.% C), that decreased to 0.7 at.% H and <0.1 at.% C when the deposition temperature was increased to 300-400 °C. Oxygen content was increasing from 47 to 56 at.% when deposition temperature was increased from 100 to 200 °C and stabilized at 56 at.% at higher temperatures ( stoichiometric value 57 at.%). As-deposited films were amorphous but crystalline MgAl2O4 with preferable (111) spinel texture was obtained when films on Si(100)  deposied at 300°C were annealed at 800–1000 °C/10 minutes in a N2 atmosphere. (Fig , XRD and AFM, 130nm MgAl2O4 film)

Deposition rate obtained with the Mg to Al precursor pulsing ratio of 1:2 was approximately 90% from the theoretical growth rate calculated from the binary oxide depositions.

According to the AFM measurements, roughness of the as-deposited films on Si(100) was significantly dependent on the pulsing ratio of the precursors, with rms increasing from 0.4 to 10 nm with increasing relative MgCp2 pulses (Fig, film thickness ~150nm, Inset shows AFM images of the Mg–Al–O films deposited with the MgCp2/H2O to AlMe3/H2O pulsing ratio of 1:2 (left) and 2:1 (right)) [125]

MgAl2O4 films have been attempted to deposit by an ALD-related process using MgCp2, AlEt3 and H2O as precursors at 370–900 °C. Deposition was successful in the temperature range of 600–750°C, limited by the low reactivity and decomposition of AlEt3. Film growth rate / verification of the self-limiting ALD deposition mechanism were however not reported. [127]

MgCp2 as a p-dopant in III-V materials

Bis(cyclopentadienyl)magnesium MgCp2 is the most commonly used p-dopant in GaN-based materials as well as GaAs, AlGaAs, InP. [[i],[ii]]

MgCp2 has been used for the p-doping of GaAs. The dependence of doping level (hole concentration) of GaAs using MgCp2, with and without AlGaAs buffer, on dopant mole fraction and growth temperature are displayed in Fig. [[iii]] Doping efficiency is dependent on the presence of oxygen/moisture in the reactor and on the reactor walls: use of AlGaAs buffer layer getters water and oxygen from the reactor walls, improving the incorporation efficiency. Mg incorporation with an AlGaAs buffer follows a square law dependence on the MgCp2 mole fraction. Mg incorporation drops rapidly with increasing growth temperature due to rapid evaporation of the dopant from the growth front.(Fig.)

MgCp2 for Mg-doped GaN layers

MgCp2 was used as precursor for the Mg doping of GaN layers grown by MOCVD on AIX 2000 HT Planetary® Reactor carrying 7x2” sapphire substrates. Mg doping concentrations of over 1·1020 cm-3 resulted in hole concentrations up to 1·1018 cm-3. p-type carrier density uniformity around 10 % (st.dev.) was observed revealing incorporation uniformity as well as influences of the activation process.[131]

GaN layers doped by MgCp2 (vaporized either as a solid or as a “Solution Cp2Mg”™), were studied by wavelength dispersive x-ray spectrometry (WDX-EPMA) as well as SIMS. No systematic differences were seen between the solid magnesocene and Solution Cp2Mg™. A ~40% reduction in Mg incorporation was observed with reduction of growth temperature from 1130 to 1090°C. No systematic structural degradation of GaN:Mg close to the magnesium solubility limit (~1020 cm−3) was observed. Up to a half of the saturation limit, [Mg] values were proportional to the magnesium source flow, and indicated magnesium atom incorporation from the gas phase with ~11% of the efficiency of gallium atoms. The most conductive sample had a hole concentration of 4.4 × 1017 cm−3, consistent with the expected generation of acceptors from only a small fraction of the magnesium atoms. [132]

Mg-doped GaN based laser diodes were grown by MOCVD using MgCp2 as a dopant. Uniformly distributed precipitates were observed by TEM in the p-type layers of laser structures; the precipitate density decreased with decreasing flow of MgCp2, which affects the hole concentrations in the p-type layers. Higher hole concentration combined with reduced precipitate density, reduced the threshold current density by 30% (from 20.8 V to 14.3 V) and improved the internal quantum efficiency due to the higher number of holes available for radiative recombination. [133]

MgCp2 was used as a dopant for the growth of Mg-doped GaN. Hole concentration was dependent on the way of dopant and reactant supply, substrate and buffer layer:  6x1018 cm-3 on sapphire by alternate pulse MOVPE technique (alternate Ga and Mg pulses in the constant NH3 flow), 2x1018 cm-3 (constant feeding of Ga and Mg sources); 2x1019cm-3 on a low temperature AlN buffer layer by using alternate feedings of Ga source and NH3 with Mg-Si co-doping, 6x1018 cm-3 on an AlGaN buffer on a SiC substrate by alternate co-doping technique. The activation energies for Mg-doped GaN grown by the pulse feedings of source materials were lower than that with continuous supply of sources.[134]

According to DLTS (deep level transient spectroscopy), deep levels measured at 0.26 and 0.62 eV below the conduction band were found in relatively low concentrations of 2x1013 cm-3 in undoped GaN Addition of small quantities of the Mg acceptor species using MgCp2 to the n-type GaN films during MOCVD growth corresponded to a significant increase in the concentration of the level at 0.62 eV. The concentration of the shallower level, found to be independent of the Cp2Mg addition, remained unchanged.[135]

When underneath p-type GaN layer was grown by MOCVD for the nitride-based LEDs with Si-doped n+-In0.23Ga0.77N/GaN short-period superlattice (SPS) tunneling contact top layer, it was found that the LED-operation voltage was almost independent of the p-dopant CP2Mg flow rate. [136]

Defects formed in the MOCVD-grown GaN doped using MgCp2 (either delta-doped or continuously doped), were studied by TEM. In the samples with Mg delta doping spontaneous ordering was observed consisting of Mg rich planar defects (showing characteristics of inversion domains) on basal planes separated by 10.4 nm and occurring only for growth in the N to Ga polar direction (000 N polarity). On the opposite site of the crystal (Ga to N polar direction), where the growth rate is an order of magnitude faster compared to the growth with N-polarity three-dimensional defects were formed: pyramidal and rectangular, empty inside with Mg segregation on internal surfaces. These types of defects were not observed in the layers where Mg was added continuously.[137]

The diffusion characteristics of Mg dopant in MOVPE-grown GaN grown on Al2O3 with Cp2Mg, TMGa and NH3 precursors, were studied.. At 1000°C – 1300°C growth temperatures the back diffusion of magnesium was observed. Abrupt diffusion profile was observed by SIMS when Cp2Mg partial pressure was increased. Applying Boltzman-Matano analysis of dopant profile it was shown that diffusion coefficient was increasing larger than one order of magnitude within the concentration range: 5x1017 - 1x1020 cm-3. Increasing growth temperature of Mg doped GaN caused shift of diffusion profiles towards layer-substrate interface.[138]

GaN-based LEDs were grown on c-plane sapphire by Low Pressure MOVPE on AIXTRON AIX200 reactor) using Cp2Mg as p-dopant for pGaN layers. Visible light emission was obserced from GaN pn-homojunctions and InGaN heterojunctions. [139]

H-atom incorporation in Mg-doped GaN grown using Cp2Mg by MOCVD was found to increase linearly with Mg concentration, suggesting the formation of simple complex between Mg and H atoms in GaN. Decrease of H-atom concentration was observed after thermal treatment in Ar, supporting the hypothesis that H-atom extraction plays an important role in obtaining low-resistivity p-type conduction [140]

MgCp2 was used as the p-dopant source for the growth of p-type GaN films on sapphire substrates in a horizontal MOCVD system. The acceptor concentration in the post-growth annealed GaN samples increased with the Mg flow rate and reached a peak value of 1x1019 cm-3 at Mg flow rate of 0.84 μmol/min. The films remained semi-insulating even after annealing when the Mg flow rate is higher than 1.08 μmol/min. Postgrowth annealing at 800°C, 30 minutes in N2 ambient is sufficient to activate most of the Mg atoms; p-type acceptor concentration obtained using rapid thermal annealing of Mg-doped GaN is comparable to the furnace annealing process. GaN LEDs using undoped layer as the n-type base layer in a p-on-n structure had light emission spectra dominated by the 430 nm peak, accompanied with two relatively weak peaks located at 380nm and 550nm. [141]

p-type Mg-doped GaN Samples with various doping concentrations were prepared by MOCVD using MgCp2 as Mg source. Variable-temperature Hall effect measurements revealed that as the dopant density was increased to the high values typically used in device applications,  the effective acceptor energy depth decreases from 190 to 112 meV, impurity conduction at low temperature becomes more prominent, the compensation ratio increases, and the valence band mobility drops sharply. The measured doping efficiency drops with Mg concentration above 2x1020 cm-3. [142]

The influence of  MgCp2 flow rate and pGaN growth temperature on electrical and optical properties of InGaN/GaN MQW LED structures was investigated. As the Cp2Mg flow rate increased at constant growth temperature, an operation voltage decreased from 5.18 to 4.34 V at 20 mA injection current. Light output power was measured as a function of injection current at room temperature; highest value was achieved at Cp2Mg flow rate of 1.06 μmol/min. LED operation voltage at RT decreased with increasing p-GaN layer growth temperature. The highest light output power measured as a function of injection current at room temperature was highest at growth temperature of 1050℃. Optimum output power-current-voltage characteristics were achieved with MgCp2 flow rate 2.13 μmol/min and 1050 ℃growth temperature. Light output power is more sensitive to growth temperature than Cp2Mg flow rates in a certain growth condition range of p-GaN:Mg layer.[143]

Low temperature photoluminescence of MgCp2-doped GaN has been investigated.  The Blue band (BB) peak of as-grown samples with higher Mg concentration is observed at lower energies, but it shifts after annealing in N2, stabilising at 2.92 eV at 850°C anneal temperature. This results has been explained by a model based on compensation eefect. [144]

 “Yellow luminescence” in PL spectra of GaN, including MgCp2-doped pGaN, was investigated. The difference between n-type and p-type doped GaN has been discussed [145]

MgCp2 as dopant for InGaN layers

MgCp2  has been applied as source for Mg doping of MOVPE InGaN (In content 0.05 to 0.37). SIMS analysis revealed that Cp2Mg memory effect as a doping source deteriorates the controllability of Mg doping level and profile, especially for thin (-0.4 μm) InGaN layers. Higher Cp2Mg flow rate was needed to get p-type conduction in InxGa1-xN with higher In content: Cp2Mg/(TEG+TMI)≈0.5% for x=0 (GaN), ≈2% for x=0.05 and ≈4% for x=0.2 (p-type conduction was obtained for In content up to 0.2). Such high Cp2Mg flow rate  was needed due to the high residual donor concentration (1019-1020 cm-3) of InGaN films and the low activation efficiency of Mg. The crystalline quality of InGaN was deteriorated with increasing In content as well as Mg doping level. To achieve a p-type InGaN with a lower Mg doping, it was essential to improve the crystalline quality of non-doped InGaN, what  was achieved by the use of a thicker GaN interlayer.[146]

MgCp2 was applied for the p-type doping of MOVPE-grown InxGa1-xN (x∼0.4) Mg-doped InGaN films with up to 23% In show p-type doping with a net acceptor concentration ~5x1016 cm−3. The sample with 37% In showed phase separation when MgCp2 flow rate is increased. By using In0.23Ga0.77N films, an n+-p homo-junctions were fabricated on a GaN template; the response to the AM1.5 illumination was observed and Voc =1.5 V and Jsc=0.5 mA/cm2 were obtained.[147]

Cp2Mg was used for the growth of Mg-doped p-InGaN layers with ~10% In by MOCVD) . Hole conc. in p-InGaN increases with increasing annealing temperature in the range 850°C. Hole mobility remains nearly unchanged, but at too high annealing temperature decreases. Using conductive p-type InGaN layers, p-In0.1Ga0.9N/i-In0.1Ga0.9N/  GaN junction structure was grown and applied for photodiodes preparation. The spectral responsivity of the InGaN/ p-i-n photodiodes shows that the peak responsivity at zero bias is in the wavelength range 350–400 nm. [148]

MgCp2 was used as p-dopant for MOCVD growth of InGaN/GaN five-period multiple quantum wells (MQWs) with different magnesium doping levels.  Magnesium doping led to a smoother surface morphology by AFM. Increase of magnesium doping concentration resulted in the decrease of V-defect density from ∼109 cm−2 (no doping) to ∼106 cm−2 (Cp2Mg: 0.04 sccm) and further to 0 (Cp2 Mg: 0.2 sccm). PL measurements indicated that magnesium doping resulted in stronger emission, which can be attributed to the screening of the polarization-induced band bending. Magnesium doping had no effect on the indium composition and MQW growth rate according to XRD. These results suggest that magnesium doping in MQWs might improve the optical properties of GaN photonic devices.[149]

Cp2Mg was applied for the growth of  Mg-doped In-rich InGaN and InN by MOCVD. Growth temp. was 550°C (1st sert), or increasing temperature with Ga content (2nd set). Upon annealing, p- InGaN was obtained from 2nd set with In content >50%, with an acceptor concentration of 1·1019 cm-3 and mobility of 1– 2 cm2/Vs. None of the samples grown at 550°C showed a p-behavior after heat treatment. [150]

MgCp2 as dopant for InN layers

Bis(cyclopentadienyl)magnesium MgCp2 has been used as a Mg source for the growth of p-doped InN layers by MOVPE. Fine grained InN(Mg) films were obtained; however those  grown at the upstream end of the 18 cm long susceptor contain a high level of C and H contamination (XPS) due to formation of adducts of MgCp2 and NH3. Adduct formation is suppressed by using optimum susceptor position from the upstream end.[151]

Cp2Mg has been tested as a Mg source for the Mg doping of InN during atmospheric-pressure MOVPE. Despite Mg content being proportional to Cp2Mg/TMI molar ratio, all layers were n-type with electron concentration increasing with increasing CP2Mg/TMI ratio. SIMS revealed that C and H contamination was reduced by optimising substrate position on the susceptor. The AFM studies showed that the grain growth of InN was suppressed by the Cp2Mg supply.[152]

MgCp2 as dopant for AlInN layers

MgCp2 was used as a p-dopant in AlInN layers grown by MOVPE (20.9% In and 219.6 arcsec FWHM by HRXRD,  sheet hole concentration of 4.73×1012 cm–2 after thermal annealing). Higher Cp2Mg flow rate decreases the conductivity of AlInN layer due to increase of Al content, but increases the surface roughness, suggesting that lower Cp2Mg flow should be used for preparation of p-type AlInN cladding layer nearly lattice-matched to GaN for optical device applications.[153]

MgCp2 as dopant for AlGaN layers

MgCp2 has been applied for the high temperature (1050°C) MOVPE growth of Mg-doped AlGaN electron-blocking barrier, used for the preparation of the blue InGaN/GaN multiple-quantum-well light-emitting diodes on sapphire substrates. The increase of Cp2Mg flow rate (50 to 200 sccm) resulted in >90 % luminescence efficiency improvement. The structures were characterised by I-V, C-V and electroluminescence measurements. SIMS revealed the Mg doping profiles close to the quantum-well active region. The increased Mg concentration resulted in improved hole injection and carrier concentration, in agreement with the increased electroluminescence efficiency and luminous intensity and with the lower turn-on voltage. [154]

High-performance Mg-doped p-type AlxGa1–xN (x = 0.35) layers were grown by LP MOCVD on AlN/sapphire template. It was found that the p-type resistivity of the AlGaN alloy demonstrates a marked dependence on the Mg concentration, V/III ratio and group III element flow rate. A minimum p-type resistivity of 3.5 Ω cm for AlxGa1–xN (x = 0.35) epilayers was achieved. [155]

Cp2Mg was used as source for the preparation of Mg-doped Al0.1Ga0.9N/undoped GaN heterostructures by MOCVD.  Two distinct hole traps were observed by deep level transient spectroscopy (DLTS): AD1 with an activation energy of 0.18 eV associated with defects in the Al0.1Ga0.9N/GaN interface, and AD2 with an activation energy of 0.88 eV strongly related to Mg-related deep levels, which contribute to the strong blue emission observed by photoluminescence measurements. The DLTS peak intensity of the AD2 trap steadily decreased with decreasing the Cp2Mg molar flow rate [156]

MgCp2 as dopant for GaAs and AlGaAs layers

Early studies reported unsuccessful use of Cp2Mg for the growth of p-doped GaAs and AlGaAs having abrupt transitions by atmospheric pressure MOVPE, as long doping tails were observed. [128]

However, later reports proved that Cp2Mg could be used as Mg p-dopant precursor for the preparation of extremely abrupt p+-n doping transitions in GaAs and AlGaAs layers grown by MOVPE. Mg incorporation was found to depend linearly on the Cp2Mg concentration, in contrast to reports of a supralinear behaviour. Methods for eliminating reactor memory effects were developed. Layers doped to ~1 x 1019 cm-3 and as thin as 100 nm at 10% of the peak doping have been grown under conditions compatible with growth of high quality Al0.3Ga0.7As.[129]

High-quality Mg-doped p-GaAs layers (p on n GaAs solar cell with an efficiency of 24% at air mass 1.5 (AM1.5)) were grown by vacuum MOCVD by using  MgCp2 purified by multiple vacuum sublimations to reduce the contamination by air and by cyclopentadiene (CP) by an order of magnitude. [130]

MgCp2 as dopant for AlGaInP layers

Cp2Mg was tested as p-doping precursor for MOCVD-grown Al0.5In0.5P used as cladding layers for 630nm band AlGaInP laser diodes, dopant concentration profile were studied. Problems of small band discontinuities and proper p-doping in terms of achieving high barriers for a reduction of thermal carrier leakage into the cladding layers in the material system (AlyGa1-y)xIn1-xP were discussed.[157]

[i] A. Xia, M.J. Heeg, C. H. Winter, Organometallics 2002, 21, 4718-4725.

[ii] A. Xia, M.J. Heeg, C. H. Winter, J. Am. Chem. Soc. 2002, 124, 11264-11265.

[iii] Thomas F. Kuech, Klavs F. Jensen, Thin Films Processes II

cladding layers in the material system (AlyGa1-y)xIn1-xP were discussed.[157]

Solution Magnesocene (MgCp2 dissolved in organic solvent)

Solution Magnesocene™

Bis(cyclopentadienyl)magnesium MgCp2, despite it is a common source for p-type doping in GaN and AlInGaP materials, has significant issues as well. First, it is a solid with a very low vapor pressure, which makes its usage in the MOCVD application more difficult. Typical issues associated with using solid Cp2Mg are channeling of carrier gas in the solid, and significant memory effects have been experienced resulting in uncontrollable doping levels.

 In order to overcome issue associated with use of solid Cp2Mg, magnesocene  diluted in an organic solvent has been developed ( „Solution Magnesocene™”. ) [SAFC, www.safchitech.com] In this source, Cp2Mg is dissolved in an organic media which is essentially nonvolatile. The advantage of this approach is a consistent delivery of the magnesium source with identical vapour pressure to Cp2Mg. The solvent (1) reduces the detrimental channeling effects associated with solid sources and (2) creates an improved delivery system by allowing more contact time between the carrier gas (hydrogen) and the magnesocene (3) reduces the memory effects by increasing transport efficiency.

The results of doping of GaN using „Solution Magnesocene™” are presented in Fig. 

Growth and comparative results of Mg-doped GaN grown by OMVPE using solid and solution Cp2Mg were presented in [158]. Using both sources, parameters were optimized to obtain high-quality GaN with hole concentrations up to 1·1018 cm3.

MgCp2 mono-thf adduct MgCp2(thf)

MgCp2 mono-thf adduct MgCp2(thf)

Mono-thf adduct of bis(cyclopentadienyl)magnesium Cp2Mg(thf) bears two η5-Cp groups, unlike other Lewis base adducts of Cp2Mg, which contain one η5-Cp group and one η1- or η2-Cp group (f.e. Cp2Mg(thf)2). This might be an advantage for higher volatility (Cp2Mg(thf) sublimes at 90°C in vacuo) and therefore for CVD applications [114]

MgCp2 adducts with amines

MgCp2 adducts with amines

Several adducts of magnesocene with amines are volatile and can be sublimed at 60°C/0.05 Torr without any evidence for reversion to magnesocene. The adduct of bis-cyclopentadienyl)magnesium with diisopropylmethylamine Cp2Mg(NH2CHiPr2) has been tested as a CVD precursor for Mg-containing films.

The solid-state structure of Cp2Mg(NH2CHiPr2) contains η5- and η2-cyclopentadienyl ligands, and the hydrogen atoms on the coordinated amine nitrogen atom participate in intramolecular and intermolecular hydrogen bonding to the η2-cyclopentadienyl ligand. The observed hydrogen bonding is relevant to the path by which cyclopentadiene is eliminated from metal cyclopentadienyl CVD source compounds during film growth employing acidic element hydrides as co-reactants.[159]

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