High C-doping of MOVPE grown thin AlxGa1−xAs layers for AlGaAs/GaAs interband tunneling devices

High hole concentrations in LP-MOVPE grown GaAs and AlGaAs layers can be achieved by intrinsic C-doping using TMGa and TMAl as carbon sources. Free carrier concentrations exceeding 10²⁰ cm⁻³ were realised at low growth temperatures between 520–540°C and V/III ratios <1.2. The C-concentration increases significantly with the Al-content in Al_xGa_1−xAs layers. We observed an increase in the atom- and free carrier concentration from 5·10¹⁹ cm⁻³ in GaAs to 1.5·10²⁰ cm⁻³ in Al_0.2Ga_0.8As for the same growth conditions. Interband tunneling devices with n-type Si and p-type C-doped AlGaAs layers and barriers made of Al_0.25Ga_0.26In_0.49P have been investigated.     

Characterization of GaxIn1−xAs grown with TMIn

High quality Ga_xIn_1−xAs, lattice matched to InP, has been reproducibly grown by organometallic vapor phase epitaxy using trimethylgallium (TMGa), trimethylindium (TMIn), and AsH₃ in an atmospheric pressure reactor with no observable adduct formation. For the first time, using TMIn, room temperature electron mobilities of 10⁴ cm²/Vs and 77 K mobilities greater than 4 × 10⁴ cm²/Vs have beep obtained. Residual donor doping densities in the low 10¹⁵ cm⁻³ range have been routinely obtained. Material with excellent morphology has been grown from 540 to 670 C with the highest quality material being obtained near 650 C. The 4 K photoluminescence (PL) peak due to carbon is not seen in the material grown at higher temperatures; however, it increases dramatically as the growth temperature is lowered. This increased carbon incorporation leads to a sharp drop in the electron mobility, which exhibits a T^−0.5 behavior between 77 and 300 K. With optimum growth conditions, 4 K PL halfwidths of 4–5 meV are commonly observed. This high quality material is characterized by x-ray diffraction, PL, and Hall mobility measurements. Carbon and other impurity incorporation as a function of the growth parameters will be described.     

Low pressure and low temperature gallium arsenide homoepitaxy employing in-situ generated arsine

Removed from the deposition region, an upstream hydrogen microwave plasma etches a surface of solid arsenic located downstream to generate arsenic hydrides. The latter are used with trimethylgallium (TMGa) to achieve low temperature (400–490° C) and low pressure (750 mTorr) homoepitaxial GaAs films. No active or afterglow plasma exists in the growth region. The homoepitaxial growth activation energy of 62 kcal/mole is consistent with the heterogeneous decomposition of TMGa in the absence of arsine. Precursor V-III ratios as low as 0.25 are used to achieve homoepitaxial films, but with high levels of carbon impurities (10¹⁹ to mid 10²⁰ cm⁻³). Carbon incorporation increases at low V-III ratios (0.25 to 0.5) for increasing temperatures with an activation energy of 23 kcal/mole. As the V-III ratios are increased above 1.0, the carbon incorporation activation energy decreases slightly to 15 kcal/mole.     

Lpe growth of Hgl−xCdxTe using conventional slider boat and effects of annealing on properties of the epilayers

The epitaxial layers of Hg_1−xCd_xTe (0.17≦×≦0.3) were grown by liquid phase epitaxy on CdTe (111)A substrates using a conventional slider boat in the open tube H₂ flow system. The as-grown layers have hole concentrations in the 10¹⁷− 10¹⁸ cm⁻³ range and Hall mobilities in the 100−500 cm²/Vs range for the x=0.2 layers. The surfaces of the layers are mirror-like and EMPA data of the layers show sharp compositional transition at the interface between the epitaxial layer and the substrate. The effects of annealing in Hg over-pressure on the properties of the as-grown layers were also investigated in the temperature range of 250−400 °C. By annealing at the temperature of 400 °C, a compositional change near the interface is observed. Contrary to this, without apparent compositional change, well-behaved n-type layers are obtained by annealing in the 250−300 °C temperature range. Sequential growth of double heterostructure, Hg_l−xCd_xTe/Hg_l−yCd_yTe on a CdTe (111)A substrate was also demonstrated.     

A comparative study of selective carbon doping in MOCVD GaAs using trimethylarsenic and arsine

Carbon incorporation in GaAs epitaxial layers grown by low pressure metalorganic chemical vapor deposition (MOCVD), using trimethylgallium (TMGa) as the gallium source and trimethylarsenic (TMAs) and AsH₃ as the arsenic sources, has been studied over a wide range of growth parameters. Carbon incorporation is identified by secondary ion mass spectroscopy (SIMS), Hall measurement, and C-V analysis. Active carbon levels between 2 × 10¹⁵ cm^-3 and 7 × 10²⁰ cm^-3 are obtained. The carbon incorporation is more sensitive to the partial pressure of TMGa than of TMAs in the growth temperature range 500 ~ 610° C. Carbon incorporation is increased as growth temperatures are decreased to 500° C for growth pressures near 10000 Pa. Results indicate that surface-adsorbed methyl radicals from the dissociation of TMGa controls carbon incorporation in this temperature range.     

Growth of (GaAs)1_x(Ge2)x by metalorganic chemical vapor deposition

We report deposition of (GaAs)_{1_x}(Ge₂)_x on GaAs substrates over the entire alloy range. Growth was performed by metalorganic chemical vapor deposition at temperatures of 675 to 750°C, at 50 and 760 Torr, using trimethylgallium, arsine, and germane at rates of 2–10 μ/h. Extrinsic doping was achieved using silane and dimethylzinc in hydrogen. Characterization methods include double-crystal x-ray rocking curve analysis, Auger electron spectroscopy, 5K photoluminescence, optical transmission spectra, Hall-effect, and Polaron profiling. Results achieved include an x-ray rocking curve full-width at half maximum as narrow as 12 arc-s, Auger compositions spanning the alloy range from x = 0.03 to x = 0.94, specular surface morphologies, and 5K photoluminescence to wavelengths as long as 1620 nm. Undoped films are n type, with n ≈ 1 × 10¹⁷ cm⁻³. Extrinsic doping with silane and dimethylzinc have resulted in films which are n type (10¹⁷ to 10¹⁸ cnr⁻³) or p type (5 × 10¹⁸ to 1 × 10²⁰ cm⁻³). Mobilities are generally ≈ 50 cm²/V-s and 500 cm²/V-s, for p and n films, respectively.     

A study of parasitic reactions between NH3 and TMGa or TMAI

The growth of AlGaN using organometallic vapor phase epitaxy has been studied as a function of reactor pressure in a horizontal reactor. At atmospheric pressure, GaN with growth efficiency comparable to that of GaAs in the same reactor is obtained. In addition, the GaN growth efficiency changes little at different reactor pressures. These results indicate that the parasitic reaction between TMGa and NH₃ is not substantial in the reactor used in this study. On the other hand, A1N growth at atmospheric pressure has not been possible. By lowering the reactor pressure below 250 Torr, A1N deposition is achieved. However, the growth efficiency decreases at higher reactor pressures and higher growth temperatures, indicating that a strong parasitic reaction occurs between TMAI and NH₃. For the ternary AlGaN, lower pressure also leads to more Al incorporation. The results indicate that parasitic reactions are much more severe for TMAI+NH3 than for TMGa+NH₃.     

Mass spectroscopy study of GaN metalorganic chemical vapor deposition

Metalorganic chemical vapor phase deposition of GaN on (100) GaAs has been studied using mass spectroscopy. With increasing substrate temperature, the amount of decomposed trimethylgallium (TMGa) was observed to increase exponentially with a characteristic energy of 1.5 eV. The presence of NH₃ was found to suppress the production of CH₃ in the gas phase. This implies that CH₃ of TMGa reacts with the hydrogen atom of NH₃, forming CH₄ as a main gas product. Studies of nitrogen evaporation from the growth surface when TMGa flow was off lead to the conclusion that increased growth rate could result in decreased background electron concentration due to nitrogen vacancy. The presence of NH₃ significantly promotes the decomposition of TMGa. Desorption of excess Ga atoms from the growth surface at low NH₃ flow rates takes place as suggested by the increased ratio of peak intensity of Ga (m/e = 69) to that of DMGa ((CH₃)₂Ga, m/e- 99) with decreasing NH₃ flow rate.     

Metalorganic vapor phase epitaxial growth of GaInAsP/GaAs

Ga_xAs_yP_1−y lattice matched to GaAs has been grown by low pressure metalorganic phase vapor epitaxy over the entire compositional range. At T_G = 670°C broad peaks of low intensity are observed in the 10K photoluminescence for y = 0.2–0.4 due to the predicted miscibility gap in this compositional region. An increase in growth temperature leads to a smaller miscibility gap. The band gap as well as the morphology show a strong dependence on substrate misorientation. The smoothest GalnAsP surfaces are obtained on exact oriented substrates. For the ternary GalnP the surface roughness is correlated to the degree of ordering in the temperature range of 600 to 750°C. The smallest band gap together with the smoothest surface is obtained on (100) 2° off to (111)B. Ordering effects are also observed in the quaternary GalnAsP. Broad-area lasers processed from the grown layers show high slope efficiency (0.9 W/A) and low internal losses (<3 cm⁻¹).     

High electron mobility in (InAs)n(GaAs)n short period superlattices grown by MOVPE for high-electron mobility transistor structure

(InAs)_n(GaAs)_n short period superlattices (SPSs) have been successfully grown by a continuous MOVPE process on InP substrates. Their structural, optical, and electrical properties have been studied. The periodic structures have been confirmed by x-ray measurements and (InAs)₁(GaAs)₁ SPSs have been clearly observed by transmission electron microscopic characterization. The optical quality of the material has been tested by 2K photoluminescence and excitonic recombinations have been observed. Mobilities as high as 10700 cm².V⁻¹.s⁻¹ and 64000 cm². V⁻¹.s⁻¹ for a sheet concentration of 3 × 10¹² cm⁻² have been obtained at 300K and 77K, respectively.     

RHEED and XPS observations of trimethylgallium adsorption on GaAs (001) surfaces—Relevance to atomic layer epitaxy

A series of experiments under UHV conditions have been performed to determine the surface reacted species and surface structure that results from trimethylgallium (TMGa) adsorption on a GaAs (001) As-stabilized surface. In these experiments, the conditions were chosen to simulate typical ALE growth conditions. Thus, the substrate temperature was varied between 320 and 530° and an admittance of 10⁻⁷ to 5 × 10⁻⁶ Torr of TMGa with exposure time of 5 to 15 sec were applied. X-ray photoelectron spectroscopy (XPS) was used to identify the chemical species on the surface after TMGa adsorption. The XPS intensity associated with the Ga 2p _3/2 level was used to monitor the quantity of adsorbed Ga and RHEED was used to monitor the surface structure. Below 440°, the Ga intensity was saturated at a level close to 1 ML and no definite Ga-stabilized 4 ×X RHEED pattern was observed. At 320° and an exposure of 200 L, a 2 × 4 As-stabilized RHEED pattern still existed, which suggests that the reaction between impinging TMGa and the (001) GaAs surface is very slow at this temperature. When the substrate temperature was between 440 and 530° exposure to greater than 6 L of TMGa resulted in saturation of surface Ga atoms to one monolayer (ML) and a successive change of surface reconstruction from 2×4 As-stabilized to 4 ×X (X = 1 or 2) Ga-stabilized surface. In all runs no carbon related species were observed within the XPS detection limit. This observation suggests that adsorption and decomposition of TMGa on As sites goes to completion very rapidly in this temperature range. From these observations we conclude that the self limiting mechanism in ALE occurs because of the differential chemisorption and decomposition rates of TMGa on As and Ga sites and that the dominant surface adsorbate is atomic Ga. This work is partially supported by the Office of Naval Research under contract No. N00014-84-K-0331 and Solar Energy Research Institute Subcontract No. XB-5-05009-3.     

An analytical evaluation of GaAs grown with commercial and repurified trimethylgallium

An analytical study of the impurities in trimethylgallium (TMGa) and subsequent correlation of the effect of these impurities on resulting GaAs films grown by metalorganic chemical vapor deposition (MOCVD) is presented. The effects of using fractional distillation techniques to improve the quality of TMGa and to help isolate and identify major source impurities in TMGa is detailed. Photothermal ionization data are presented which show the residual donor species present and their relative concentrations in the epitaxial layers. Correlations of the residual donor concentrations with TMGa preparation are made. It is demonstrated that high purity GaAs with μ₇₇ K ≈ 125,000 cm²/V-sec can be grown by MOCVD using repurified trimethylgallium and arsine source materials. Work supported in part by the U.S. Naval Research Laboratory on Contract No. N00173-80-C-0066.     

Molecular beam epitaxial growth of P-ZnSe:N using a novel plasma source

We have studied the p-type doping in ZnSe molecular beam epitaxial growth using a novel high-power (5 kW) radio frequency (rf) plasma source. The effect of growth conditions such as the rf power, the Se/Zn flux ratio and the growth temperature on p-ZnSe:N was investigated. The net acceptor concentration (N_A—N_D) of around 1 × 10¹⁸ cm⁻³ was reproducibly achieved. The activation ratio ((N_A—N_D)/[N]) of p-ZnSe:N with N_A—N_D of 1.2 × 10¹⁸ cm⁻³ was found to be as high as 60%, which is the highest value so far obtained for N_A—N_D ∼ 10¹⁸ cm⁻³. The 4.2K photoluminescence spectra of p-ZnSe:N grown under the optimized growth condition showed well-resolved deep donor-acceptor pair emissions even with high N_A—N_D. On leave from Sumitomo Electric Industry Ltd. On leave from Sony Corp.     

Anomalous anisotropy in the absorption coefficient of vacancy-ordered Ga2Se3

The structural and optical properties of Ga₂Se₃ on (100)GaP and (100)GaAs prepared by molecular beam epitaxy have been investigated. The electron diffraction studies revealed that the superstructure was formed in [011] direction by the spontaneous ordering of native gallium-vacancies in the defect zinc blende structure under a selenium-rich growth condition, and very large absorption anisotropy (Δα>10⁴cm¹) was observed in the vacancy-ordered Ga₂Se₃. Furthermore, polarization dependence of photoconductivity due to the absorption anisotropy was observed in the vacancy-ordered Ga₂Se₃ on (100)GaAs.     

Raman study of defects in a GaAs buffer layer grown by low-temperature molecular beam epitaxy

Raman scattering measurements on high-resistivity layers of GaAs grown by molecular beam epitaxy at low temperature are presented. Several defect-related features are ob-served, including two peaks attributed to quasi-localized vibrational modes of point de-fects, one with a frequency of 223 cm⁻¹ similar to a mode previously observed in elec-tron-irradiated GaAs, and the other with a frequency of 47 cm⁻¹ similar to a mode observed in ion-implanted GaAs. We suggest that these are due to arsenic interstitials and gallium vacancies, respectively. We also observe peaks at 200 and 258 cm⁻¹, which we believe may be due to vibrational modes in small clusters of arsenic. The 223 cm⁻¹ mode is the only defect-related mode still observed after a 10-min annealing treatment at 600° C, although it is significantly broader and has different symmetry from the 223 cm⁻¹ mode in the unannealed material. This indicates that the 223 cm⁻¹ mode in the annealed material is due, at least in part, to a defect other than the arsenic interstitial.     

Carbon reduction in triethylarsenic-grown GaAs films using chemically activated arsine as a co-reagent

N-type gallium arsenide films grown from triethylarsenic (Et₃As) and trimethylgallium (Me₃Ga) are generally of poor quality (μ_77K(max) = 16,100 cm²/V-s) and are severely contaminated with carbon (>10¹⁸ cm⁻³), whereas films grown using a mixture of triethylarsenic and arsine (AsH₃) with Me₃Ga are typically of high purity (β_77K(max) = 60,000 cm²/V-s) and contain significantly reduced carbon levels (~mid-10¹⁵ cm^-3). These differences in film purity are due to the inherent growth chemistry of each reagent mixture. The respective growth chemistries of these reagent systems have been inferred from a series of decomposition experiments carried out under pseudo-growth conditions, and the differences in growth chemistry are consistent with the differences in corresponding epilayer purity. Triethylarsenic appears to decompose primarily via a bond homolysis reaction to generate alkyl-containing radical species, which can react with a growing GaAs epilayer to cause severe carbon contamination. In the Et₃As/AsH₃ coreagent system, the Et₃As reagent decomposes to produce these alkyl-containing radical intermediates, but they then apparently react further with the arsine co-reagent to generate reactive arsenic hydride radicals under relatively facile conditions. These reactive arsenic-hydride radical species can contribute to the GaAs growth process without introducing carbon into the resultant films.     

Improved interfacial properties of GaAs MOS capacitor with NH3-plasma-treated ZnON as interfacial passivation layer

The GaAs MOS capacitor was fabricated with HfTiON as high-k gate dielectric and NH₃-plasma-treated ZnON as interfacial passivation layer (IPL), and its interfacial and electrical properties are investigated compared to its counterparts with ZnON IPL but no NH₃-plasma treatment and without ZnON IPL and no plasma treatment. Experimental results show that low interface-state density near midgap (1.17×10¹² cm^-2eV^-1) and small gate leakage current density have been achieved for the GaAs MOS device with the stacked gate dielectric of HfTiON/ZnON plus NH₃-plasma treatment. These improvements could be ascribed to the fact that the ZnON IPL can effectively block in-diffusion of oxygen atoms and out-diffusion of Ga and As atoms, and the NH₃-plasma treatment can provide not only N atoms but also H atoms and NH radicals, which is greatly beneficial to removal of defective Ga/As oxides and As-As band, giving a high-quality ZnON/GaAs interface.     

Optical Properties of Nanocrystalline GaN Films Prepared by Annealing Amorphous GaN in Ammonia

Nanocrystalline GaN films were prepared by thermal treatment of amorphous GaN films under flowing NH₃ at a temperature of 600°C to 950°C for 1 h to 2 h. X-ray diffraction and field-emission scanning electron microscopy confirmed the formation of high-crystal-quality hexagonal GaN films with preferential (002) orientation. The photoluminescence spectrum showed a sharp peak near the band gap emission located at 368 nm and a broad blue peak centered at 430 nm. Five first-order Raman modes near ∼143 cm⁻¹, 535 cm⁻¹, 555 cm⁻¹, 568 cm⁻¹, and 731 cm⁻¹ with two new additional Raman peaks at 257 cm⁻¹ and 423 cm⁻¹ were observed. The origin of these new Raman peaks is discussed briefly.     

Growth and properties of Hg1-x Cdx Te epitaxial layers

The growth of epitaxial layers of mercury-cadmium-telluride (Hg_1-xCd_xTe) with relatively low x (0.2-0.3) from Te-rich solutions in an open tube sliding system is studied. The development of a semiclosed slider system with unique features permits the growth of low x material at atmospheric pressure. The quality of the films is improved by the use of Cd_1-yZ_yTe and Hg_1-xCd_xTe substrates instead of CdTe. The substrate effects and the growth procedure are discussed and a solidus line at a relatively low temperature is reported. The asgrown epitaxial layers are p-type with hole concentration of the order of 1·10¹⁷ cm⁻³, hole mobility of about 300 cm²·V⁻¹ sec⁻¹ and excess minority carrier life-time of 3 nsec, at 77 K.     

Carbon tetrachloride doped Al x Ga1−x As grown by metalorganic chemical vapor deposition

A dilute mixture of CCl₄ in H₂ has recently been shown to be a suitable carbon doping source for obtainingp-type GaAs grown by metalorganic chemical vapor deposition (MOCVD) with carbon acceptor concentrations in excess of 1 × 10¹⁹cm⁻³. To understand the effect of growth parameters on carbon incorporation in CCl₄ doped Al _x Ga_1−x As, carbon acceptor concentration was studied as a function of Al composition, growth temperature, growth rate, and CCl₄ flow rate using electrochemical capacitance-voltage profiling. The carbon incorporation as a function of Al composition, growth temperature and CCl₄ flow rate was also measured by secondary ion mass spectroscopy (SIMS). All layers were grown by low pressure MOCVD using TMGa and TMAl as column III precursors, and 100% AsH₃ as the column V source. Increased Al composition reduced the dependence of carbon concentration on the growth temperature. Reduced growth rate, which resulted in substantially decreased carbon acceptor concentrations in GaAs, had an insignificant effect on the carrier concentration of Al_0.4Ga_0.6As. A linear relationship between hole concentration and CC1₄ flow rate in Al_xGa_1−x As for 0.0 ≤x ≤ 0.8 was observed. These results are interpreted to indicate that adsorption and desorption of CCl _y (y ≤ 3) on the Al _x Ga_1-x As surface during crystal growth plays an important role in the carbon incorporation mechanism.