Growth of AlGaN epitaxial layers and AlGaN/GaN superlattices by metal-organic chemical vapor deposition

Special features of metal-organic chemical vapor deposition of AlGaN epitaxial layers and AlGaN/GaN superlattices either in an Epiquip VP-50 RP research and development reactor (for a single wafer 2 in. in diameter) or in an AIX2000HT production-scale reactor (for up to six wafers 2 in. in diameter) are stud-ied. It is found that the dependence of the aluminum content in the solid phase on the trimethylaluminum (TMA) flux in a reactor levels off; this effect hinders the growth of the layers with a high aluminum content in both types of reactors and is more pronounced in the larger reactor (AIX2000HT). Presumably, this effect is a consequence of spurious reactions in the vapor phase and depends on the partial pressure of TMA in the reactor. The aluminum content in the layers can be increased not only by reducing the total pressure in the reactor but also by increasing the total gas flow through the reactor and reducing the trimethylgallium flux. The approaches described above were used to grow layers with a mole fraction of AlN as large as 20% in the AIX2000HT production-scale reactor at a pressure of 400 mbar (this fraction was as large as 40% at 200 mbar). AlGaN layers with the entire range of composition were grown in the Epiquip VP-50 RP reactor.     

Interface structural defects and photoluminescence properties of epitaxial GaN and AlGaN/GaN layers grown on sapphire

Overall characterization of the GaN and AlGaN/GaN epitaxial layers by X-ray diffractometry and optical spectral analysis is carried out. The layers are grown by metalloorganic gas-phase epitaxy on (0001)-oriented single crystal sapphire wafers. The components of strains and the density of dislocations are determined. The effects of strains and dislocations on the photoluminescence intensity and spectra are studied. The results allow better understanding of the nature and mechanisms of the formation of defects in the epitaxial AlGaN/GaN heterostructures.     

Study of the polarization photoluminescence of thick epitaxial GaN layers

Polarization photoluminescence spectra of gallium nitride were obtained. It follows from an analysis of the spectra that inhomogeneous broadening of the emission line having a half-width greater than 20 meV can be determined from the dispersion of the angles θ _c of the symmetry axes of the crystallites forming the epitaxial GaN layer, relative to the surface of the layer. Varying the angle of incidence, the focusing of the exciting laser beam, and the photoluminescence recording angle makes it possible to use polarization photoluminescence measurements for precision diagnostics of the quality of GaN layers. Fiz. Tekh. Poluprovodn. 33, 778–780 (July 1999)     

Electrical domains and submillimeter signal generation in AlGaN/GaN superlattices

This paper discusses the feasibility of a terahertz-signal source made of an AlGaN/GaN superlattice. The negative differential conductivity, electrical domain formation, current oscillations, and power efficiency of a perspective source are described. The superlattice geometry and conduction band profile, which are distorted by polarization fields, are related to the oscillation frequency and power efficiency of the device. The optimal Al content, superlattice period, and the parameters of the external circuit that favor submillimeter wave generation are determined.     

P- and N-type doping of GaN and AlGaN epitaxial layers grown by metalorganic chemical vapor deposition

Mg- and Si-doped GaN and AlGaN films were grown by metalorganic chemical vapor deposition and characterized by room-temperature photoluminescence and Hall-effect measurements. We show that the p-type carrier concentration resulting from Mg incorporation in GaN:Mg films exhibits a nonlinear dependence both on growth temperature and growth pressure. For GaN and AlGaN, n-type doping due to Si incorporation was found to be a linear function of the silane molar flow. Mg-doped GaN layers with 300K hole concentrations p ∼2×10¹⁸ cm⁻³ and Si-doped GaN films with electron concentrations n∼1×10¹⁹ cm⁻³ have been grown. N-type Al_0.10Ga_0.90N:Si films with resistivities as low as p ∼6.6×10⁻³ Ω-cm have been measured.     

Influence of temperature and drain current on source and drain resistances in AlGaN/GaN HEMTs

The behavior of source and drain resistances (R_S and R_D) has been studied for a wide range of drain currents at ambient temperatures from 150 to 500 K. Both parasitic resistances show an important increase as temperature rises, directly related to the reduction in the electron mobility. High drain currents also produce a non-linear increment of R_S and R_D, once the space-charge limited current is exceeded. Both temperature and drain current mechanisms have been modeled together by means of a simple equation, and a good agreement between simulations and measurements is found. Non-linear R_S and R_D allow a more accurate extraction of the intrinsic parameters, especially in the high drain current range. The use of variable parasitic resistances instead of their usually assumed constant values reveals higher intrinsic transconductance (g_m,int) and C_gs.     

Influence of AlGaN deep level defects on AlGaN/GaN 2-DEG carrierconfinement

We have used low energy electron-excited nanoscale luminescence spectroscopy (LEEN) to detect the defects in each layer of AlGaN/GaN HEMT device structures and to correlate their effect on two-dimensional electron gas (2-DEG) confinement. We investigated AlGaN/GaN heterostructures with different electrical properties using incident electron beam energies of 0.5 to 15 keV to probe electronic state transitions within each of the heterostructure layers. AlGaN heterostructures of 25 nm thickness and nominal 30% Al concentration grown on GaN buffer layers on sapphire substrates by plasma-assisted molecular beam epitaxy exhibited a range of polarization-induced electron densities and room temperature mobilities. In general, the spectra exhibit AlGaN band edge emission at ~3.8 eV or ~4.0 eV, GaN band edge emission at ~3.4 eV, yellow luminescence (YL) features at 2.18 eV and 2.34 eV, and a large emission in the infrared (<1.6 eV) from the GaN cap layer used to passivate the AlGaN outer surface. These heterostructures also show high strain in the 2 nm-thick GaN layer with evidence for a Franz-Keldysh red shift due to piezoelectric charging. The LEEN depth profiles reveal differences between the structures with and without 2-DEG confinement and highlight the importance of AlGaN defects in the near 2-DEG region     

AlGaN/GaN和AlGaN SBD的温度特性研究

研究了AlGaN/GaN异质结构上的肖特基接触的基本原理及载流子的高温输运特性.将AlGaN/GaN异质结SBD和AlGaN SBD,在27～250℃进行实验比较.发现随着温度上升,AlGaN SBD的势垒高度下降,理想因子增加,其影响因素包括热电子发射、场发射、隧穿效应及复合电流效应等机制.而AlGaN/GaN异质结SBD由于受到压电极化场和2DEG和的影响,其势垒高度和理想因子随温度的变化趋势与AlGaNSBD相反.实验结果还显示,AlGaN/GaN异质结SBD的反向电流随着温度的上升,呈现先增大后减小的趋势.     

On the temperature and carrier density dependence of electron saturation velocity in an AlGaN/GaN HEMT

The temperature and carrier density dependence of electron intrinsic saturation velocity (v/sub si/) in a 0.3-/spl mu/m gate length AlGaN/GaN HEMT was extracted from multibias S-parameter measurements. It was found that v/sub si/ fell rapidly with increasing sheet carrier concentration (n/sub s/), but was only a very weak function of ambient temperature (T/sub amb/). This behavior is consistent with the hot-phonon model of carrier transport.     

Effect of ion dose and annealing mode on photoluminescence from SiO2 implanted with Si ions

This paper discusses the photoluminescence spectra of 500-nm-thick layers of SiO₂ implanted with Si ions at doses of 1.6×10¹⁶, 4×10¹⁶, and 1.6×10¹⁷ cm⁻² and then annealed in the steady-state region (30 min) and pulsed regime (1 s and 20 ms). Structural changes were monitored by high-resolution electron microscopy and Raman scattering. It was found that when the ion dose was decreased from 4×10¹⁶ cm⁻² to 1.6×10¹⁶ cm⁻², generation of centers that luminesce weakly in the visible ceased. Moreover, subsequent anneals no longer led to the formation of silicon nanocrystallites or centers that luminesce strongly in the infrared. Annealing after heavy ion doses affected the photoluminescence spectrum in the following ways, depending on the anneal temperature: growth (up to ∼700 °C), quenching (at 800–900 °C), and the appearance of a very intense photoluminescence band near 820 nm (at >900 °C). The last stage corresponds to the appearance of Si nanocrystallites. The dose dependence is explained by a loss of stability brought on by segregation of Si from SiO₂ and interactions between the excess Si atoms, which form percolation clusters. At low heating levels, the distinctive features of the anneals originate predominantly with the percolation Si clusters; above ∼700 °C these clusters are converted into amorphous Si-phase nanoprecipitates, which emit no photoluminescence. At temperatures above 900 °C the Si nanocrystallites that form emit in a strong luminescence band because of the quantum-well effect. The difference between the rates of percolation and conversion of the clusters into nanoprecipitates allows the precipitation of Si to be controlled by combinations of these annealings. Fiz. Tekh. Poluprovodn. 32, 1371–1377 (November 1998)     

Electrical properties of silicon layers implanted with erbium and oxygen ions in a wide dose range and thermally treated in different temperature conditions

The electrical properties of silicon implanted with Er and O ions in a wide dose range have been studied. The dependence of electron mobility on the concentration of electrically active centers is determined for Si:Er layers with Er concentrations in the range of 9×10¹⁵–8×10¹⁶ cm^?3. Sharp bends related to specific features of Er segregation in solid-phase epitaxial recrystallization are observed in the concentration profiles of electrically active centers, n(x), and Er atoms, C(x), at Er ion implantation doses exceeding the amorphization threshold. The n(x) and C(x) profiles virtually coincide near the surface. A linear rise in the maximum concentration of electrically active centers at approximately constant effective coefficient of their activation, k, is observed at Er implantation doses exceeding the amorphization threshold. At an Er concentration higher than 7×10¹⁹ cm^?3, the concentration of electrically active centers levels off and k decreases.     

Improved temperature model of AlGaN/GaN HEMT and device characteristics at variant temperature

In this paper, an improved temperature model for AlGaN/GaN high electron mobility transistor (HEMT) is presented. Research is being conducted for a high-performance building block for high frequency applications that combine lower costs with improved performance and manufacturability. The effects of channel conductance in the saturation region and the parasitic resistance due to the undoped GaN buffer layer have been included. The effect of both spontaneous and piezoelectric polarization induced charges at the AlGaN/GaN heterointerface has been incorporated. The proposed model is used to determine the transfer characteristics, output current-voltage characteristics and small-signal microwave parameters of HEMTs. The investigated temperature range is from 100–600 K. The small signal microwave parameters have been evaluated to determine the unity current gain cut-off frequency (f _T ). High f _T (10–70 GHz) values and high current levels (～550 mA/mm) are achieved for a 1 μm AlGaN/GaN HEMTs. A custom DC measurement system is used to facilitate the DC characterization of the unpackaged GaN HEMT test device. The calculated critical parameters and the simulation results suggest that the performance of the proposed device degrades at elevated temperatures.     

Effect of charged dislocation walls on mobility in GaN epitaxial layers

A theoretical model in the context of a conventional representation on traditional notion concerning Read cylinders for interpretation of mobility collapse as a function of the concentration of free carriers in GaN-based films is suggested. Along with phonon and impurity scattering mechanisms, electron scattering due to charged dislocations embedded into the walls is taken into account in the model. An expression is obtained for the height of the drift barrier depending on the concentration of free carriers. Based on the derived equations, the dependence of the location of the mobility minimum on the dislocation structure is interpreted.     

Measurement of temperature in active high-power AlGaN/GaN HFETsusing Raman spectroscopy

We report on the noninvasive measurement of temperature, i.e., self-heating effects, in active AlGaN/GaN HFETs grown on sapphire and SiC substrates. Micro-Raman spectroscopy was used to produce temperature maps with ≈1 μm spatial resolution and a temperature accuracy of better than 10°C. Significant temperature rises up to 180°C were measured in the device gate-drain opening. Results from a three-dimensional (3-D) heat dissipation model are in reasonably good agreement with the experimental data. Comparison of devices fabricated on sapphire and SiC substrates indicated that the SiC substrate devices had ~5 times lower thermal resistance     

表面态对AlGaN/GaN异质结构中二维电子气的影响

基于静电学分析,得出表面态是电子的一个重要来源.基于这一分析,可以解释已发表的关于二维电子气(2DEC)的大量数据.例如,2DEG密度随着AlGaN层厚度、Al组分的变化的原因.当A10.3Ga0.7N/GaN结构中生长一层5 nm厚的GaN冒层时,2DEG浓度由1.47×1013cm-2减少到1.20×1013cm-2,减少是由于表面类施主态离化减少.由于充分厚的GaN冒层导致GaN/AlGaN/GaN上界面形成二维空穴气(2DHG),所以在超出特定的冒层厚度时2DEG浓度达到饱和.     

Influence of defects on the photoluminescence kinetics in GaN/AlN quantum-dot structures

The influence of defects in the AlN barrier on photoluminescence decay after pulse excitation is studied for structures with GaN quantum dots in an AlN matrix. For these quantum-dot structures, it is found that the initial part of the decay curves corresponds to fast photoluminescence decay. Comparison of the photoluminescence-decay curves for the GaN/AlN quantum-dot structures and AlN layers without quantum dots shows that fast decay is defined by the contribution of the photoluminescence band related to defects in the AlN matrix.     

Influence of Field Plate on the Transient Operation of the AlGaN/GaN HEMT

In this letter, a link between the AlGaN/GaN high-electron-mobility-transistor (HEMT) field plate (FP) and the rate of reoccupation of surface traps is presented. Surface traps are considered to be among the primary factors behind HEMT performance deterioration at high frequencies. Results from simulations using the commercial software package DESSIS are presented, in which the FP is found to reduce trap reoccupation by limiting the tunneling injection of electrons into surface traps in the gate-drain region and thus considerably improve the transient operation of the device.     

Specific features of the mechanisms of excitation of erbium photoluminescence in epitaxial Si:Er/Si structures

The excitation spectra and kinetics of erbium photoluminescence and silicon interband photoluminescence in Si:Er/Si structures under conditions of high-intensity pulse optical excitation are studied. It is shown that, in the interband photoluminescence spectra of the Si:Er/Si structures, both the luminescence of free excitons and the emission associated with the electron-hole plasma can be observed, depending on the excitation power and wavelength. It is found that the formation of a peak in the erbium photoluminescence excitation spectra at high pumping powers correlates with the Mott transition from the exciton gas to the electron-hole plasma. It is demonstrated that, in the Si:Er/Si structures, the characteristic rise times of erbium photoluminescence substantially depend on the concentration of charge carriers.     

Influence of the Source–Gate Distance on the AlGaN/GaN HEMT Performance

In this paper, we present Monte Carlo simulation results on the source-gate (S-G) scaling effects in GaN-based HEMTs. The results show that a downscaling of the S-G distance can improve device performance, enhancing the output current and the device transconductance. The main reason for this effect is related to the peculiar dynamic of electrons in the GaN-based HEMTs, which leads to a nonsaturated velocity regime in the source access region, even for high drain applied voltages. On the contrary, the gate-drain distance does not affect the output current within the analyzed device geometries. Based on these results, new optimization strategies for GaN HEMTs could be defined     

Effect of a trace of water vapor on Ohmic contact formation for AlGaN/GaN epitaxial wafers

The authors observed that a trace of water vapour can have a significant degradation effect on Ohmic contact formation for AlGaN/GaN high electron mobility transistors (HEMTs). The degradation effect due to a trace of water vapour is less serious for n-GaN. This is a more serious problem for AlGaN/GaN HEMT than AlGaAs/GaAs HEMT or InP based HEMT because of the higher temperature needed for Ohmic contact annealing when AlGaN or GaN are involved. Using cooling water with a temperature slightly higher than the ambient temperature during rapid thermal annealing (RTA) appears to be the best approach.