Double Heterostructure Devices Research Articles

The III-Nitride Double Heterostructure Revisited: Benefits for Threshold Voltage Engineering of MIS Devices

GaN-based devices are seen as ideal candidates for power-switching applications. For the acceptance of GaN-based devices by module designers, obtaining enhancement-mode (e-mode) behavior in GaN-based heterostructure field-effect transistors (HFETs) has long been in the focus. Although the gate-injection approach appears to be the most promising one to achieve e-mode devices, using a double heterostructure in conjunction with a gate insulator has still its advantages, such as steeper turn-ON characteristics and lower leakage currents. An analytical expression to predict the threshold voltage $V_{\mathrm {th}}$ for a given double heterostructure device has not yet been derived. Moreover, neither an evaluation of the tradeoff between $V_{\mathrm {th}}$ and ON-state resistance $R_{{\text {ds}},\mathrm {\scriptstyle {{ON}}}}$ has been performed to date. This paper addresses these two aspects. We will show an analytical expression for a metal–insulator–semiconductor double HFET (MIS-DHFET), which in certain cases is also valid for the gate-injection transistor. On the basis of this, we will discuss the actual influence of the Al concentration in the backbarrier on $V_{\mathrm {th}}$ . We will further employ technology computer aided design (TCAD) device simulations to evaluate the impact on $R_{{\text {ds}},\mathrm {\scriptstyle {{ON}}}}$ when using MIS-DHFETs. It will be shown that by implementing a double heterostructure in MIS devices, it is possible to suppress the typically observed negative $V_{\mathrm {th}}$ –oxide thickness relationship while maintaining a constant $R_{{\text {ds}},\mathrm {\scriptstyle {{ON}}}}$ .

Enhancement mode AlGaN/GaN double heterostructure high electron mobility transistor with F plasma treatment

Effects of double heterostructure materials (AlGaN/GaN/AlGaN/GaN) with different GaN channel thickness values (14 nm, 28 nm, 60 nm) on the high electron mobility transistor (HEMT) are simulated by using silvaco, and furthermore, the differences in characteristic among the enhancement mode devices made from such double heterostructure materials with different F injection doses (150 W, 135 W) are also simulated. The simulation results show that the threshold voltage shifts towards positive direction and the saturation current decreases as the GaN channel thickness decreases. The two-dimensional electron gas (2 DEG) density could be reduced as GaN channel thickness decreases due to piezoelectric polarization weakened by backing AlGaN barrier. Combining F plasma treatment and double heterostructure material, the enhancement mode device with high positive threshold voltage is successfully developed. The DC characteristics of the enhancement mode devices with different GaN channel thickness values are analyzed comparatively, and the simulation results are validated by using the experimental results. The threshold voltages of these enhancement mode devices with GaN channel thickness values of 14 nm, 28 nm, and 60 nm reach 1.1 V, 0.8 V, and 0.3 V, respectively. The maximum transconductance values of these enhancement mode devices with GaN channel thickness values of 14 nm, 28 nm, and 60 nm reach 115 mS/mm, 137 mS/mm, and 198 mS/mm, respectively. The thinner GaN channel thickness in the double heterostructure could reduce the depth of quantum well and 2 DEG density, so that the device with a GaN channel thickness of 14 cm has a lower saturation current. The breakdown voltages and gate reverse leakage currents of the three kinds of devices are investigated, and the device with a thinner GaN channel has a lower leakage current and higher breakdown voltage due to weakened vertical electrical field in thinner channel double heterostructure. The damage of channel mobility in F plasma treatment is weakened by using a lower plasma power (135 W), and the enhancement mode device without annealing process demonstrates a better saturation current and transconductance characteristic. The results of the device with annealing confirm that the plasma damage is depressed at an F injection power of 135 W. The threshold voltage temperature stability of 14 nm GaN channel thickness device is studied, and Vth is only 0.4 V after 350 ℃ 2 min annealing process. Drain induced barrier lowering (DIBL) effects of the HEMTs with double heterostructures are investigated, and the DIBL value of the14 nm GaN channel device is 16 mV/V. The DIBL value indicates a good limiting property of the 2 DEG in double heterostructure device.

Single- and double-heterostructure GaN-HEMTs devices for power switching applications

This paper reports on a detailed characterization of single- (SH) and double-heterostructure (DH) GaN-based HEMTs developed for the employment on power switching applications. DC, pulsed and breakdown characterizations have been performed in five different epitaxial structures. SH devices present higher ID current, however, DH devices present much lower leakage current and higher breakdown voltage, thanks to the better confinement provided by the back barrier layer. Kink effect have also been observed in these devices and the effect is enhanced in double heterostructure devices. Off-state step-stress, carried out on a sub-set of the available devices, revealed good reliability performances of DH devices. The limited reliability of the SH devices could be related to the punch-through leakage current. Correlation between current degradation and electroluminescence hot-spot in electroluminescence (EL) measurements has also been found.

Buffer-Related Degradation Aspects of Single and Double-Heterostructure Quantum Well InAlN/GaN High-Electron-Mobility Transistors

We experimentally prove the viability of the concept of the double-heterostructure quantum well InAlN/GaN high-electron-mobility transistor (HEMT) for the device higher robustness and reliability. In the single quantum well InAlN/GaN HEMTs, the intrinsic channel resistance increases by 300% after 1 h off-state stress; much less degradation is observed in the double-heterostructure device with an AlGaN back barrier. Physics-based device simulation proves that the back barrier blocks the rate of carrier injection into the device buffer. However, whatever the quantum well design is, the energy of the injected electrons in the buffer of InAlN/GaN-based HEMTs is higher than that in the buffer of AlGaN/GaN HEMTs. This energy may be sufficient for releasing hydrogen from GaN point defects.

High breakdown voltage in AlGaN/GaN/AlGaN double heterostructures grown on 4 inch Si substrates

AbstractWe report high breakdown combined with low on‐resistance results on AlGaN/GaN/AlGaN double heterostructure devices (DHFET) grown on 4 inch Silicon substrates. On two fingers devices with gate‐drain distance (LGD) of 8 μm we measured a breakdown voltage (VBD) of 650 V and an on‐resistance (Ron) of 7.2 Ω·mm. We also demonstrated that increasing the buffer thickness and etching deep into the AlGaN buffer layer improve both buffer and device breakdown. Moreover, we have also achieved a high breakdown voltage (580 V) for smaller LGD (5 μm) combined with the lowest reported on‐resistance (6 Ω·mm) for this voltage range. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Double heterostructure ultraviolet light emitting diodes with nanometer scale compositionally inhomogeneous active regions

AbstractWe report on the fabrication and evaluation of flip‐chipped double heterostructure UVLEDs operating at 320 nm that employ a bulk (80nm) AlGaN active region containing nanoscale compositional inhomogeneities (NCI), deposited by plasma‐assisted molecular beam epitaxy. The devices were deposited on AlGaN templates grown by hydride vapor phase epitaxy and contain electron and hole injection layers grown by metalorganic chemical vapor deposition. A packaged, 300 μm x 300 μm device with 3% internal quantum efficiency has a peak output power of 0.56 mW at 90 mA DC current that corresponds to an external quantum efficiency of 0.15%. Nearly constant external quantum efficiency is observed for DC current density up to 100 A/cm2, suggesting that the NCI regions effectively suppress the deleterious effects of polarization fields at low injection currents that should inhibit radiative recombination in double heterostructure devices. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Current versus voltage characteristics of GaN∕AlGaN∕GaN double heterostructures with varying AlGaN thickness and composition under hydrostatic pressure

We have studied current versus voltage characteristics of n-GaN∕u-AlGaN∕n-GaN double heterostructure devices under hydrostatic pressure up to 500MPa. Devices were grown on c-plane sapphire substrates by organometallic vapor phase epitaxy using epitaxial layer overgrowth. The effect of AlGaN layer thickness and composition on the pressure sensitivity was investigated. For a fixed applied bias, we found that the current decreases approximately linearly in magnitude with increasing hydrostatic pressure over the range of voltages and pressures applied. The decrease in current magnitude can be attributed to piezoelectric effects and is consistent with model calculations. The polarization charge densities at the GaN∕AlGaN interfaces change with hydrostatic pressure, which in turn modifies the internal potential barrier. Changes in the AlGaN layer thickness and composition also modify the interfacial polarization, with thicker AlGaN layers and higher AlN content increasing the effect of pressure on the observed current versus voltage characteristics. The strain gauge factors obtained for these devices range from ∼200 to 800.

Electroluminescent device with biphenylpyrazine derivative Eu salt LB (Langmuir–Blodgett) film as a molecular-size emissive layer

Using the Eu salt Langmuir–Blodgett (LB) film of a biphenylpyrazine derivative (C4BPy), we fabricated a double-heterostructure device with molecular-size emissive layer, ITO/diamine hole-transporting layer/ Eu salt LB film of C4BPy (two monolayers)/oxadiazole electron-transporting layer/MgAg. When the DH device was driven at 170 K, only sharp red emission due to the Eu salt bimolecular layer was observed, and no emission due to the HTL and the ETL was observed. The result demonstrates that the C4BPy Eu salt bimolecular layer was successfully actuated as molecular-size emissive layer. The external quantum efficiency and maximum luminance of the device were 0.01% and 3 cd/m 2, respectively.

Laser action in organic semiconductor waveguide and double-heterostructure devices

Stimulated emission by optical pumping of solid-state organic materials has been well known since the late 1960s following thefirst demonstrations of laser action in dye-doped gels and molecular crystals1,2,3,4. Interest in this field has been revived by the demonstration of efficient, long-lived and intense electroluminescence in both polymeric5 and small-molecular-weight6 organic thin films, which indicates the possibility of laser action in these materials. Several recent studies of optically pumped polymers have reported emission phenomena suggestive of laser action7,8,9. Here we present clear evidence for laser action from optically pumped, vacuum-deposited thin films of organic molecules, in both slab-waveguide and double-heterostructure configurations. This realization of laser action in conducting organic thin films should open the way to the development of a new class of electrically pumped laser diodes.

II–VI light-emitting devices based on beryllium chalcogenides

Abstract Beryllium chalcogenides are semiconductors with a large band gap. They have a higher-degree of covalent bonding than all of the other II–VI compounds. The bond energies of BeTe, BeSe and BeS are comparable to those of GaN. Due to their band gaps and their lattice constants they can be incorporated in quaternary mixed crystals which are lattice matched to GaAs. The hardness of II–VI materials containing beryllium offers new possibilities to enlarge the lifetime of laser diodes, emitting in the green and blue spectral range. First light-emitting diodes (LEDs) with beryllium as a constituent have been produced on GaAs substrates by molecular beam epitaxy (MBE). Double heterostructure devices were realized with quantum wells either of BeZnSeTe, ZnSe or BeZnSe. Special ohmic contacts consisting of BeTeZnSe graded superlattices were employed. Although for the first devices the growth conditions were not optimized, the lifetime at typical operating currents was very promising.

Growth of InAsSb quantum wells for long-wavelength (∼4 μm) lasers

Molecular-beam epitaxy (MBE) has been employed for the growth of quantum-well laser structures grown on GaSb substrates. These lasers consist of compressively strained InAsSb wells, tensile-strained InAlAsSb barriers, and lattice-matched AlAsSb cladding layers. A broad-stripe diode laser with an emission wavelength of 3.9 μm has operated cw up to 123 K, with a characteristic temperature T0 of 30 K. Optical pumping of this laser structure has provided pulsed operation up to 162 K, with a peak output power of up to ∼0.6 W at 110 K. Emission wavelengths as long as 4.5 μm have been obtained in other laser structures with InAlAs barriers. In this article we present the key issues regarding the MBE growth of long-wavelength strained quantum-well lasers on GaSb, including shutter sequencing for interface control, analysis of characterization data, and laser results, with a comparison to double-heterostructure devices.

Lasing operation up to 200 K in the wavelength range of 570–590 nm by GaInP/AlGaInP double-heterostructure laser diodes on GaAsP substrates

Short-wavelength stimulated emission from a GaInP/AlGaInP double-heterostructure (DH) grown on GaAs0.6P0.4 substrates, where lattice-matched Ga0.7In0.3P is the active layer with Γ band-gap energy beyond 2.1 eV is investigated. Laser oscillation is attained at a wavelength below 590 nm. This shows that the DH attains sufficient carrier confinement for lasing even though the minimum Γ band-gap energy is close to that in the X band. By applying high-reflectivity coating on both facets of the cavity to decrease the optical mirror loss, we achieve lasing operation by the DH devices under pulsed current injection up to 200 K. The device exhibits threshold currents of 115 mA at 77 K and 380 mA at 200 K, and an output power level up to 0.3 mW. The oscillation wavelength is 577 nm at 77 K and 588 nm at 200 K when the current is injected at 1.2 times the threshold.

Cavity length dependence of photopumped lasing properties of MOCVD-grown ZnSe/ZnMgSSe double-heterostructure

An internal loss of 4.6 cm-1 is estimated from the cavity length dependence of photopumped blue lasing at room temperature (RT) with a ZnSe/ZnMgSSe double-heterostructure (DH) grown by metal organic chemical vapour deposition (MOCVD). The low internal loss, being comparable to that of molecular beam epitaxy (MBE)-grown DH, implies the feasibility of realising an MOCVD-grown DH device.

Spin-Dependent Transport in GaN Light Emitting Diodes

ABSTRACTElectrically detected magnetic resonance (EDMR) is used to study recombination processes in two types of gallium nitride light emitting diodes: in m/i/n/n+- and InGaN/AlGaN double-heterostructure devices. In the MIS-diodes, two resonances at g=1.96 and 2.00, corresponding to the effective mass donor and a deep defect are observed at room temperature. At low temperatures, an acceptor-related resonance at g=2.06 is visible as well. After current degradation, the spectra are dominated by the defect resonance, indicating that the creation of this defect is responsible for the decreased electroluminescence efficiency. In the double-heterostrucrure devices, EDMR can only be observed below 60 K showing the g=2.00 defect resonance. The same defect resonance is also observed in conventional electron spin resonance experiments under illumination (light-induced ESR).

Theoretical study of the effect of an AlGaAs double heterostructure on metal-semiconductor-metal photodetector performance

The impulse and square-wave input response of different GaAs metal-semiconductor-metal photodetector (MSM) designs are theoretically examined using a two dimensional drift-diffusion numerical calculation with a thermionic-field emission boundary condition model for the heterojunctions. The rise time and the fall time of the output signal current are calculated for a simple GaAs, epitaxially grown, MSM device as well as for various double-heterostructure barrier devices. The double heterostructure devices consist of an AlGaAs layer sandwiched between the top GaAs active absorption layer and the bottom GaAs substrate. The effect of the depth of the AlGaAs layer on the speed and responsivity of the MSM devices is examined. It is found that there is an optimal depth, at fixed applied bias, of the AlGaAs layer within the structure that provides maximum responsivity at minimal compromise in speed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Minority carrier lifetimes in molecular beam epitaxy grown AlxGa1−xAs/GaAs double heterostructures doped with aluminum

Both Al-treated pBN crucibles and Al-doped Ga melts have been used to reduce Ga-cell related oval defect densities in molecular beam epitaxy (MBE) grown epilayers. This practice, although effective in reducing the oval defect density, results in small amounts of Al contamination in the grown films. In this work, we have grown diagnostic Al0.3Ga0.7As/GaAs double heterostructure (DH) devices doped with Al in order to determine the effect of Al contamination on the minority-carrier lifetime. The active region of the DH structures was doped with Al at 5×1018 atoms/cm3 to emulate the levels observed when Al-treated pBN crucibles are used. Al impurity levels and distribution profiles were characterized by secondary ion mass spectrometry, and minority-carrier lifetimes of DH devices were characterized using time-resolved photoluminescence (PL) measurements. Device structures grown with various GaAs active layer thicknesses were used to calculate the interface recombination velocity (S) and the bulk lifetime (τB). We found that Al doping of the GaAs active layer adversely affects the measured PL decay time. GaAs active layers of DH devices doped with Al exhibited poor bulk lifetimes (τB≤120 ns) limited by Shockley–Read–Hall recombination. In contrast, DH devices grown with Al-free active layers resulted in significantly improved PL decay times with bulk lifetimes as high as τB=700 ns.

Light Emission from Quantum Well Structures Containing ZnS, ZnSe, and Related Alloys

Properties of blue and blue-green laser diodes with ZnCdSe single- and multiple-quantum-well active regions are discussed. Blue, blue-green, and green light emitting diodes which employ ZnCdSe or ZnSTeSe as the active region of double heterostructure devices have also been prepared and tested. Properties of ZnS-ZnSSe and ZnS-ZnCdS quantum well structures are also reported. These structures emit intense photoluminescence in the ultraviolet spectral region.

Confinement of charge carriers and molecular excitons within 5-nm-thick emitter layer in organic electroluminescent devices with a double heterostructure

Organic electroluminescent devices with a double-heterostructure indium-tin-oxide substrate/hole transport layer/emitter layer/electron transport layer/MgAg have been fabricated by vacuum vapor deposition. The organic carrier transport and emitter layers were composed of amorphous films. In the double-heterostructure devices, the luminance continued to lie in high level, even when the emitter thickness was 50 Å. The confinement of charge carriers and molecular excitons within a narrow emitter layer was achieved.

Tradeoff between phase- and intensity modulation in GaAs/AlGaAs double heterostructure and multiple quantum well phase-modulator waveguides

The wavelength dependence of phase sensitivity ( degrees /mm V) and intensity modulation of a double-heterostructure (DH) device and a multiquantum-well (MQW) device in the GaAs/AlGaAs material system is studied. The results show the tradeoff between phase modulation and intensity modulation and also show that, for the same intensity modulation, the MQW device produces twice as much phase modulation as the DH device. For example, at an intensity modulation of -1 dB the MQW device has a phase sensitivity of 120 degrees /mm V while the DH device gives a value of approximately=60 degrees /mm V. >

GaAs‐ALGaAs double‐heterostructure coupled‐waveguide optical modulators and their high‐frequency modulation characteristics

AbstractIn this paper, a GaAs‐AlGaAs double‐heterostructure (DH) coupled‐waveguide optical modulator, which is important as a multifunction optical component and for possible monolithic integration, has been studied in detail. A DH device with pn‐junction structure, which is expected to have the lowest switching voltage, has been fabricated and the optical modulation characteristics have been measured at 1.06 μm wavelength. At a reverse bias of 13 V, an extinction ratio of 11 dB has been obtained. However, the cutoff frequency has been limited to about 250 MHz, which was lower than the calculated value of 1.1 GHz obtained using a simple lumped‐element circuit analysis. Therefore, the optical modulation characteristics have been analyzed assuming the device as a distributed‐element circuit line, and also taking into account the external parasitic impedancees. Thus it has been concluded that a reduction of the top‐electrode resistance and an increase of the conductance across the device cross‐section are necessary to improve the high‐frequency optical modulation characteristics.To verify these results, a DH Schottky contact structure, which essentially possesses no ohmic contact resistance with a thick topelectrode, has been fabricated on a MBE‐grown wafer. At a reverse bias of 16 V, an extinction ratio of 12 dB and a cut‐off frequency of 1.5 GHz have been obtained successfully. It has also been concluded that an even thicker top‐electrode and an improved mounting method which minimizes the external parasitic elements are necessary for further improvement of the optical modulation frequency characteristics.