Electroreflectance spectra of InGaN/AlGaN/GaN quantum-well heterostructures

p?n InGaN/AlGaN/GaN heterostructures with InGaN/AlGaN multiple quantum wells are studied by electroreflectance spectroscopy. The structures are grown by metal—organic epitaxy and arranged with the p region in contact with the heat sink. Light is incident on and reflected from the structures through the sapphire substrate. To modulate the reflectivity, rectangular voltage pulses and a dc reverse bias are applied to the p?n junction. A line corresponding to interband transitions in the region of InGaN/AlGaN multiple quantum wells is observed in the electroreflectance spectra. The peak of this line is shifted to shorter wavelengths from the peak of injection luminescence of the light-emitting diode structures. The low-field model developed by Aspnes is used to describe the electroreflectance spectra. By choosing the parameters of the model to fit the experimental data, the effective band gap of the active region of the structure, E _g ^* , is determined at 2.76–2.78 eV. The experimental dependence of E _g ^* on the applied voltage is attributed to the effect of piezoelectric fields in the InGaN quantum wells. In the electroreflectance spectra, an interference pattern is observed in the wide spectral range from 1.4 to 3.2 eV. The interference is due to the dependence of the effective refractive index on the electric field.     

Interference effects in the electroreflectance and electroluminescence spectra of InGaN/AlGaN/GaN light-emitting-diode heterostructures

Interference effects in InGaN/AlGaN/GaN light-emitting-diode heterostructures of blue emission are studied by spectroscopy of electroreflectance and electroluminescence. The periodic bands observed in the electroreflectance and electroluminescence spectra in a blue spectral range are caused by interference effects in the structure in general. The emergence of interference fringes in the electroreflection spectra is explained by modulation of built-in electric fields in the active region of the heterostructure. The long-period interference fringes observed in the electroreflectance spectra in a wide spectral range from infrared to ultra-violet allows one to determine the location of the active region of the heterostructure with respect to different reflecting surfaces in the cavity.     

Luminescence spectra of blue and green light-emitting diodes based on multilayer InGaN/AlGaN/GaN heterostructures with quantum wells

The luminescence spectra of blue and green light-emitting diodes based on In_xGa_1−x N/Al_yGa_1−y N/GaN heterostructures with a thin (2–3 nm) In_xGa_1−x N active layer have been investigated in the temperature and current intervals 100–300 K and J=0.01–20 mA, respectively. The spectra of the blue and green light-emitting diodes have maxima in the interavals ℏω_max=2.55–2.75 eV and ℏω_max=2.38–2.50 eV, respectively, depending on the In content in the active layer. The spectral intensity of the principal band decreases exponentially in the long-wavelength region with energy constant E ₀=45–70 meV; this is described by a model that takes into account the tails of the density of states in the two-dimensional active region and the degree of filling of the tails near the band edges. At low currents radiative tunneling recombination with a voltage-dependent maximum in the spectrum is observed in the spectra of the blue diodes. A model of the energy diagram of the heterostructures is discussed. Fiz. Tekh. Poluprovodn. 31, 1055–1061 (September 1997)     

Anisotropy in the quantum lifetime in AlGaN/GaN heterostructures

Magnetoresistance measurements were performed on van der Pauw shaped AlGaN/GaN heterostructures grown on either sapphire or silicon carbide. These measurements revealed the presence of Shubnikov-de Haas oscillations. However, the amplitude of the oscillations originating from perpendicular van der Pauw positions were not isotropic. This anisotropy varied from sample to sample and within a sample its magnitude changed with the carrier density which was modulated by illumination as it induced a persistent photocurrent. The results of this study suggest the anisotropy is either a manifestation of electron density inhomogeneities and/or an indication of a nonuniform scattering mechanism arising from nonuniform interface roughness.     

Lasing in the vertical direction in InGaN/GaN/AlGaN structures with InGaN quantum dots

InGaN/GaN structures with dense arrays of InGaN nanodomains were grown by metallorganic chemical vapor deposition. Lasing in vertical direction occurs at low temperatures, indicating ultrahigh gains (～ 10⁵ cm^?1) in the active region. Fabrication of an effective AlGaN/GaN distributed Bragg reflector with reflectivity exceeding 90% enables vertical lasing at room temperature in structures with a bottom distributed Bragg reflector, despite the absence of a well-reflecting upper mirror. The lasing wavelength is 401 nm, and the threshold excitation density is 400 kW/cm².     

氮空位在纳米GaN颗粒和InGaN/AlGaN双异质结中的聚集

直流放电等离子法制备纳米GaN颗粒中的氮缺乏可导致空位形成。在电子显微观察的电子辐照条件下，这些N-空位将进一步凝聚，形成一个a=2.209nm,b=3.826nm,c=1.037nm,α＝β＝γ＝90℃ 的调制结构。随着电子辐照剂量增加，纳米颗粒中心将出现空洞，同时使该区的金属镓离子迁移到颗粒的表面。电子显微分析及分子力学理论计算表明，这种新的调制结构系空位的有序排列所致。在此基础上，进一步研究了InGaN/AlGaN的双异质结薄膜结构中直径约为50nm的空洞存在与发光失效的关系，讨论了N－空位的聚集与空形成的关系。     

The influence of composition and unintentional doping on the two-dimensional electron gas density in AlGaN/GaN heterostructures

We have studied the influence of Al content, AlGaN layer thickness, and unintentional background doping by oxygen on the two-dimensional electron gas (2DEG) density in AlGaN/GaN heterostructures. Hall measurements were made on samples grown with molecular beam epitaxy. The 2DEG densities in the range 2–3×10¹³ cm^?2 were measured. A one-dimensional Schrödinger-Poisson model was used to describe the heterostructure. The calculations gave two-dimensional electron densities in accordance with measured values. The electron density is very sensitive to the Al concentration in the AlGaN layer, whereas the sensitivity to layer thickness is small. Our simulations also showed that the two-dimensional concentration increased 50% when the free-carrier concentration changed from 10¹⁵ cm^?3 to 10¹⁸ cm^?3. The relation between donor concentration and free-carrier concentration was found to agree when using oxygen ionization energy as a parameter.     

Electron scattering in AlGaN/GaN heterostructures with a two-dimensional electron gas

The temperature and concentration dependences of electron mobility in AlGaN/GaN hetero-structures are studied. The mobility for the samples under study at T = 300 K lies in the range of 450–1740 cm²/(V s). It is established that scattering at charged centers is dominant for samples with low mobility (lower than 1000 cm²/(V s) right up to room temperature. These centers are associated with a disordered piezoelectric charge at the heterointerface because of its roughness or with a piezoelectric charge similarly to the Al-GaN barrier because of alloy disorder, as well as with the deformation field around dislocations. Scattering at optical phonons is dominant for samples with mobility exceeding 1000 cm²/(V s) at T = 300 K. Scattering at alloy disorders, heterointerface roughness, and dislocations are dominant at temperatures lower than 200 K. A decrease in the influence of scattering at roughness with improvement of the heterointerface morphology increases room-temperature mobility from 1400 cm²/(V s) to 1700 cm²/(V s).     

InGaN/GaN quantum well growth on pyramids of epitaxial lateral overgrown GaN

InGaN/GaN quantum wells (QW) were grown by metalorganic chemical vapor deposition (MOCVD) on pyramids of epitaxial lateral overgrown (ELO) GaN samples. The ELO GaN samples were grown by MOCVD on sapphire (0001) substrates that were patterned with a SiN_x mask. Scanning electron microscopy and cathodoluminescence (CL) imaging experiments were performed to examine lateral variations in structure and QW luminescence energy. CL wavelength imaging (CLWI) measurements show that the QW peaks on the top of the grooves are red-shifted in comparison with the QW emission from the side walls. The results show that In atoms have migrated to the top of the pyramids during the QW growth. The effects of V/III ratio, growth temperature as well as ELO GaN stripe orientation on the QW properties are also studied.     

Electrical properties and luminescence spectra of light-emitting diodes based on InGaN/GaN heterostructures with modulation-doped quantum wells

The distribution of charged centers N(w), quantum efficiency, and electroluminescence spectra of blue and green light-emitting diodes (LED) based on InGaN/AlGaN/GaN p-n heterostructures were investigated. Multiple InGaN/GaN quantum wells (QW) were modulation-doped with Si donors in GaN barriers. Acceptor and donor concentrations near the p-n junction were determined by the heterodyne method of dynamic capacitance to be about N _A ≥ 1 × 10¹⁹ cm^?3 ? N _D ≥ 1 × 10¹⁸ cm^?3. The N(w) functions exhibited maxima and minima with a period of 11–18 (±2–3 nm) nm. The energy diagram of the structures has been constructed. The shifts of spectral peaks with variation of current (J=10^?6–3×10^?2 A) are smaller (13–12 meV for blue and 20–50 meV for green LEDs) than the corresponding values for the diodes with undoped barriers (up to 150 meV). This effect is due to the screening of piezoelectric fields in QWs by electrons. The dependence of quantum efficiency on current correlates with the charge distribution and specific features in the current-voltage characteristics.     

Transport coefficients of AlGaN/GaN heterostructures

We have experimentally determined the effective mass (m*) of GaN, the classical (τ _c), and quantum (τ _q) scattering times for a two-dimensional electron gas residing at the interface of an AlGaN/GaN heterostructure, using the Shubnikovde Haas effect. The ratio of the two scattering times, τ _c/τ _q, suggests that, at low temperatures, the scattering mechanism limiting the mobility is due to remote ionized impurities located in AlGaN. This study should provide sample growers with information useful for improving the quality of the nitride heterostuctures.     

Impact ionization luminescence of InGaN/AlGaN/GaN p-n-heterostructures

The luminescence spectra of InGaN/AlGaN/GaN p-n heterostructures with reverse bias sufficient for impact ionization are investigated. The injection luminescence of light-emitting diodes with such structures was examined earlier. A strong electric field is present in the InGaN active layer of the heterostructures, and for small reverse bias the tunneling component of the current predominates. Avalanche breakdown commences at voltages V _th>8–10 V, i.e., ∼3E _g , (E _g is the width of the band gap) in the absence of lightly doped structures. The luminescence spectra have a short-wavelength edge corresponding to the width of the GaN band gap (3.40 eV) and maxima in the region 2.60–2.80 eV corresponding to the maxima of the injection luminescence spectra in the active layer. The long-wavelength edge of the spectra in the region 1.7–1.8 eV may be associated with deep recombination levels. Mechanisms of recombination of the hot electron-hole plasma in the strong electric fields of the p-n heterostructures are discussed. Fiz. Tekh. Poluprovodn. 32, 63–67 (January 1998)     

Exciton states in wurtzite and zinc-blende InGaN/GaN coupled quantum dots

Based on the effective-mass approximation, exciton states in wurtzite (WZ) and zinc-blende (ZB) InGaN/GaN coupled quantum dots (QDs) are studied by means of a variational method. Numerical results show clearly that both the sizes and In content of QDs have a significant influence on exciton states in WZ and ZB InGaN/GaN coupled QDs. Moreover, the ground-state exciton binding energy decreases when the interdot barrier layer thickness increases in the WZ InGaN/GaN coupled QDs. However, the ground-state exciton binding energy has a minimum if the interdot barrier layer thickness increases in the ZB InGaN/GaN coupled QDs.     

A 357.9 nm GaN/AlGaN multiple quantum well ultraviolet laser diode

Ultraviolet(UV)and deep-UV light emitters are prom-ising for various applications including bioagent detection,wa-ter and air purification,dermatology,high-density optical stor-age,and lithography.However,to achieve shorter UV laser di-odes(LDs),especially for the LDs with lasing wavelength less than 360 nm,requires high AlN mole fraction AlGaN clad-ding layer(CL)and waveguide(WG)layers,which usually leads to generate cracks in AlGaN epilayer due to lack of lat-tice-matched substrates.Meanwhile,due to high resistivity of high AlN mole fraction Mg doped AlGaN layers,only few groups have reported GaN-based LDs with emission wavelength shorter than 360 nm[1?8],and up to now,there is no room temperature continuous-wave(CW)operation UV LDs with a lasing wavelength shorter than 360 nm ever repor-ted.Previously,we have reported a UV LD with lasing wavelength of 366 nm[9].In this paper,a higher AlN mole frac-tion of AlGaN WG layers is employed to shorten the LD emis-sion wavelength to less than 360 nm.A lasing wavelength of 357.9 nm is achieved.     

Investigation of radiative tunneling in GaN/InGaN single quantum well light-emitting diodes

The mechanisms of carrier injection and recombination in a GaN/InGaN single quantum well light-emitting diodes have been studied. Strong defect-assisted tunneling behavior has been observed in both forward and reverse current–voltage characteristics. In addition to band-edge emission at 400 nm, the electroluminescence has also been attributed to radiative tunneling from band-to-deep level states and band-to-band tail states. The approximately current-squared dependence of light intensity at 400 nm even at high currents indicates dominant nonradiative recombination through deep-lying states within the space-charge region. Inhomogeneous avalanche breakdown luminescence, which is primarily caused by deep-level recombination, suggests a nonuniform spatial distribution of reverse leakage in these diodes.     

DC and RF Characteristics of AlGaN/GaN/InGaN/GaN Double-Heterojunction HEMTs

We present the detailed dc and radio-frequency characteristics of an Al_0.3Ga_0.7N/GaN/In_0.1Ga_0.9 N/GaN double-heterojunction HEMT (DH-HEMT) structure. This structure incorporates a thin (3 nm) In_0.1Ga_0.9N notch layer inserted at a location that is 6-nm away from the AlGaN/GaN heterointerface. The In_0.1Ga_0.9N layer provides a unique piezoelectric polarization field which results in a higher potential barrier at the backside of the two-dimensional electron gas channel, effectively improving the carrier confinement and then reducing the buffer leakage. Both depletion-mode (D-mode) and enhancement-mode (E-mode) devices were fabricated on this new structure. Compared with the baseline AlGaN/GaN HEMTs, the DH-HEMT shows lower drain leakage current. The gate leakage current is also found to be reduced, owing to an improved surface morphology in InGaN-incorporated epitaxial structures. DC and small- and large-signal microwave characteristics, together with the linearity performances, have been investigated. The channel transit delay time analysis also revealed that there was a minor channel in the InGaN layer in which the electrons exhibited a mobility slightly lower than the GaN channel. The E-mode DH-HEMTs were also fabricated using our recently developed CF₄-based plasma treatment technique. The large-signal operation of the E-mode GaN-based HEMTs was reported for the first time. At 2 GHz, a 1times100 mum E-mode device demonstrated a maximum output power of 3.12 W/mm and a power-added efficiency of 49% with single-polarity biases (a gate bias of +0.5 V and a drain bias of 35 V). An output third-order interception point of 34.7 dBm was obtained in the E-mode HEMTs     

InGaN/GaN multiple quantum well green light-emitting diodes prepared by temperature ramping

High-quality InGaN/GaN multiple-quantum well (MQW) light-emitting diode (LED) structures were prepared by a temperature-ramping method during metal-organic chemical-vapor deposition (MOCVD) growth. Two photoluminescence (PL) peaks, one originating from well-sensitive emission and one originating from an InGaN quasi-wetting layer on the GaN-barrier surface, were observed at room temperature (RT). The observation of high-order double-crystal x-ray diffraction (DCXRD) satellite peaks indicates that the interfaces between InGaN-well layers and GaN-barrier layers were not degraded as we increased the growth temperature of the GaN-barrier layers. With a 20-mA and 160-mA current injection, it was found that the output power could reach 2.2 mW and 8.9 mW, respectively. Furthermore, it was found that the reliability of the fabricated green LEDs prepared by temperature ramping was also reasonably good.     

Luminescence and electrical properties of InGaN/AlGaN/GaN light emitting diodes with multiple quantum wells

Luminescence spectra of light-emitting diodes based on InGaN/AlGaN/GaN heterostructures with multiple quantum wells are studied for currents in the range J=0.15 μA-150 mA. The comparatively high quantum efficiency for low J(J _max=0.5–1 mA) is a consequence of a low probability for the nonradiative tunnel current. The current-voltage characteristics J(V) are studied for J=10⁻¹²–10⁻¹ A; they are approximated by the function V=ϕk+mkT· [1n(J/J ₀)+(J/J ₁)^0.5] + J · R _s. The portion of V∞(J/J ₁)^0.5 and measurements of the dynamic capacitance indicate that i-layers adjacent to the active layer play an important role. The spectra are described by a model with a two-dimensional density of states with exponential tails in multiple quantum wells. The rise in T with increasing J is determined from the short-wavelength decay of the spectrum of the blue diodes: T=360–370 K for J=80–100 mA. An emission band is observed at 2.7–2.8 eV from green diodes at high J; this band may be explained by phase separation with different amounts of In in the InGaN. Fiz. Tekh. Poluprovodn. 33, 445–450 (April 1999)     

Analysis of Interface Scattering in AlGaN/GaN/InGaN/GaN Double-Heterojunction High-Electron-Mobility Transistors

This paper presents detailed investigations on the direct-current (DC) characteristics of an AlGaN/GaN/InGaN/GaN double-heterojunction high-electron-mobility transistor (DH-HEMT) using two-dimensional numerical analysis. In this work, the hot-electron effect is taken into account and implemented in the hydrodynamic model. The results indicate that carrier transport in this kind of device exhibits properties significantly different from that in a conventional AlGaN/GaN HEMT. Due to imperfections at the GaN/InGaN interface, scattering caused by the interface roughness, phonons, etc. inhibit the negative differential conductance in high electric field. In addition, the velocity increment of electrons around the gate edge is dominated by the overshoot effect rather than the phonon effect. The energy exchange between phonons and electrons, as presented in this paper, illustrates that the dissipated power is just a small portion of the exchanged energy. For further performance improvement, more lattice-matched material with strong polarization for the barrier layer is proposed.     

阱宽和垒厚对lnGaN/GaN多量子阱结构光电特性的影响

通过求解修正的基于K·p方法的有效质量哈密顿方程并与泊松方程进行自洽,得到在极化效应影响下的不同阱宽和垒厚的InGaN/GaN多量子阱导带和价带的能带结构,并计算了不同多量子阱结构的自发辐射谱.仿真结果表明:阱宽和垒厚对InGaN/GaN多量子阱结构的光电子学特性有很大的影响.随着阱宽和垒厚的增加,InGaN/GaN多...