Nitride-based p-i-n bandpass photodetectors

Nitride-based p-i-n bandpass photodetectors with semitransparent Ni-Au electrodes were successfully fabricated and characterized. The photodetectors exhibit a 20-V breakdown voltage and a small dark current of 40 pA at 4-V reverse bias. It was found that spectral responsivity shows a narrow bandpass characteristics from 337 to 365 nm. Moreover, the peak responsivity was estimated to be 0.13 A/W at 354 nm, corresponding to a quantum efficiency of 44%. The relatively high response at shorter wavelength is due to the unoptimized thickness of p-Al/sub 0.1/Ga/sub 0.9/N absorption layer. At low frequency, the noise of the photodetector is dominant by the 1/f-type noise. For our 330/spl times/330 /spl mu/m/sup 2/ device, given a bias of -3.18 V, the corresponding noise equivalent power and normalized detectivity D/sup */ are calculated to be 5.6/spl times/10/sup -12/ W and 3.34/spl times/10/sup 11/ cmHz/sup 0.5/ W/sup -1/, respectively.     

GaN MSM UV photodetectors with titanium tungsten transparent electrodes

GaN metal-semiconductor-metal (MSM) ultraviolet photodetectors with titanium tungsten (TiW) transparent electrodes were fabricated and characterized. It was found that the 10-nm-thick TiW film deposited with a 300-W RF power can still provide a reasonably high transmittance of 75.1% at 300 nm, a low resistivity of 1.7/spl times/10/sup -3/ /spl Omega//spl middot/cm and an effective Schottky barrier height of 0.773 eV on u-GaN. We also achieved a peak responsivity of 0.192 A/W and a quantum efficiency of 66.4% from the GaN ultraviolet MSM photodetector with TiW electrodes. With a 3-V applied bias, it was found that minimum noise equivalent power and maximum D/sup */ of our detector were 1.987/spl times/10/sup -10/ W and 6.365/spl times/10/sup 9/ cmHz/sup 0.5/W/sup -1/, respectively.     

A resonant tunneling quantum-dot infrared photodetector

A novel device-resonant tunneling quantum-dot infrared photodetector-has been investigated theoretically and experimentally. In this device, the transport of dark current and photocurrent are separated by the incorporation of a double barrier resonant tunnel heterostructure with each quantum-dot layer of the device. The devices with In/sub 0.4/Ga/sub 0.6/As-GaAs quantum dots are grown by molecular beam epitaxy. We have characterized devices designed for /spl sim/6 /spl mu/m response, and the devices also exhibit a strong photoresponse peak at /spl sim/17 /spl mu/m at 300 K due to transitions from the dot excited states. The dark currents in the tunnel devices are almost two orders of magnitude smaller than those in conventional devices. Measured values of J/sub dark/ are 1.6/spl times/10/sup -8/ A/cm/sup 2/ at 80 K and 1.55 A/cm/sup 2/ at 300 K for 1-V applied bias. Measured values of peak responsivity and specific detectivity D/sup */ are 0.063 A/W and 2.4/spl times/10/sup 10/ cm/spl middot/Hz/sup 1/2//W, respectively, under a bias of 2 V, at 80 K for the 6-/spl mu/m response. For the 17-/spl mu/m response, the measured values of peak responsivity and detectivity at 300 K are 0.032 A/W and 8.6/spl times/10/sup 6/ cm/spl middot/Hz/sup 1/2//W under 1 V bias.     

A dependency of quantum efficiency of silicon CMOS n/sup +/pp/sup +/ LEDs on current density

A dependency of quantum efficiency of nn/sup +/pp/sup +/ silicon complementary metal-oxide-semiconductor integrated light-emitting devices on the current density through the active device areas is demonstrated. It was observed that an increase in current density from 1.6/spl times/10/sup +2/ to 2.2/spl times/10/sup +4/ A/spl middot/cm/sup -2/ through the active regions of silicon n/sup +/pp/sup +/ light-emitting diodes results in an increase in the external quantum efficiency from 1.6/spl times/10/sup -7/ to 5.8/spl times/10/sup -6/ (approximately two orders of magnitude). The light intensity correspondingly increase from 10/sup -6/ to 10/sup -1/ W/spl middot/cm/sup -2//spl middot/mA (approximately five orders of magnitude). In our study, the highest efficiency device operate in the p-n junction reverse bias avalanche mode and utilize current density increase by means of vertical and lateral electrical field confinement at a wedge-shaped n/sup +/ tip placed in a region of lower doping density and opposite highly conductive p/sup +/ regions.     

AlGaN solar-blind Schottky photodiodes fabricated on 4H-SiC

Solar-blind AlGaN-based Schottky photodiodes grown on 4H-SiC substrate are reported. The fabricated devices demonstrate dark current density as low as 2.2/spl times/10/sup -10/ A/cm/sup 2/ at a reverse bias of 5 V. A zero-bias peak responsivity of 44 mA/W was achieved at 256 nm, corresponding to an external quantum efficiency of 21%. Under a low illumination power density of 10 nW/cm/sup 2/, a rejection ratio of more than two orders of magnitude was observed in the wavelength range from 270 to 310 nm. A room-temperature solar-blind detectivity of 7.9/spl times/10/sup 14/ cm/spl middot/Hz/sup 1/2/W/sup -1/ was estimated at 256 nm under zero bias.     

Photoresponse of (In,Ga)N-GaN multiple-quantum-well structures in the visible and UVA ranges

Characterization and analysis of photoresponse in p-n diodes with embedded (In,Ga)N-GaN multiple-quantum-well (MQW) structures are reported. Their dependence on the number of wells and In composition are considered. The influence of device structure on electric fields in the active region and on device responsivity has also been studied. Theoretical considerations as well as photocapacitance and photocurrent measurements show that the position of quantum wells (QWs), either in the quasi-neutral region or in the space charge region, is a critical factor in the collection efficiency. Hence, device photoresponse is not proportional to the number of QWs in photovoltaic mode. Present p-MQW-n devices show a promising performance as UVA and visible photodetectors, with detectivities, D/sup */, higher than 1.2/spl times/10/sup 12/ cm/spl middot/Hz/sup 1/2//spl middot/W/sup -1/ and rejection ratios higher than 10/sup 3/.     

Interdigitated Ge p-i-n photodetectors fabricated on a Si substrate using graded SiGe buffer layers

We report an interdigitated p-i-n photodetector fabricated on a 1-/spl mu/m-thick Ge epitaxial layer grown on a Si substrate using a 10-/spl mu/m-thick graded SiGe buffer layer. A growth rate of 45 /spl Aring//s/spl sim/60 /spl Aring//s was achieved using low-energy plasma enhanced chemical vapor deposition. The Ge epitaxial layer had a threading dislocation density of 10/sup 5/ cm/sup -2/ and a rms surface roughness of 3.28 nm. The 3-dB bandwidth and the external quantum efficiency were measured on a photodetector having 1-/spl mu/m finger width and 2-/spl mu/m spacing with a 25/spl times/28 /spl mu/m/sup 2/ active area. At a wavelength of 1.3 /spl mu/m, the bandwidth was 2.2, 3.5, and 3.8 GHz at bias voltages of -1, -3, and -5 V, respectively. The dark current was 3.2 and 5.0 /spl mu/A at -3 and -5 V, respectively. This photodetector exhibited an external quantum efficiency of 49% at a wavelength of 1.3 /spl mu/m.     

Observation of lasing from vertically self-organized InAs three-dimensional island quantum boxes on GaAs (001)

We report the first observation of lasing from vertically self-organized multiple stacks of electronically uncoupled InAs three-dimensional island quantum boxes grown via molecular beam epitaxy on GaAs (001) substrates. A low-threshold current density of 310 A/cm/sup 2/ at 79 K is found for a stack of five sets of islands corresponding to 2 ML InAs depositions separated by 36 monolayer GaAs spacers grown via migration enhanced epitaxy. The distribution of the island volumes (1.5/spl times/10/sup 5/ /spl Aring//sup 3/-4 /spl times/10/sup 5/ /spl Aring//sup 3/) gives, as expected, a multitude of laser lines between 980 mm and 996 nm.     

GaAs MOSFET using InAlP native oxide as gate dielectric

GaAs metal-oxide-semiconductor field-effect transistors (MOSFETs) using wet thermally oxidized InAlP as the gate insulator are reported for the first time. Leakage current measurements show that the 11-nm-thick native oxide grown from an In/sub 0.49/Al/sub 0.51/P layer lattice-matched to GaAs has good insulating properties, with a measured leakage current density of 1.39/spl times/10/sup -7/ mA//spl mu/m/sup 2/ at 1 V bias. GaAs MOSFETs with InAlP native gate oxide have been fabricated with gate lengths from 7 to 2 /spl mu/m. Devices with 2-/spl mu/m-long gates exhibit a peak extrinsic transconductance of 24.2 mS/mm, an intrinsic transconductance of 63.8 mS/mm, a threshold voltage of 0.15 V, and an off-state gate-drain breakdown voltage of 21.2 V. Numerical Poisson's equation solutions provide close agreement with the measured sheet resistance and threshold voltage.     

Low-resistance ultrashallow extension formed by optimized flash lamp annealing

Flash lamp annealing (FLA) technology is proposed as a new method of activating implanted impurities. By optimizing FLA and implantation conditions, junction depth (Xj) at the concentration of 1 /spl times/ 10/sup 18/ cm/sup -3/ and the sheet resistance of 13 nm and 700 /spl Omega//sq for As and 14 nm and 770 /spl Omega//sq for BF/sub 2/ with junction leakage lower than 1 /spl times/ 10/sup -16/ A//spl mu/m/sup 2/ at 1.5 V were successfully obtained without wafer slip and warpage problems.     

Development of liquid cooling techniques for flip chip ball grid array packages with High Heat flux dissipations

In this paper, development of single-phase liquid cooling techniques for flip chip ball grid array packages (FBGAs) with high flux heat dissipations is reported. Two thermal test chips with different footprints, 12 mm/spl times/ 12 mm and 10 mm /spl times/10 mm, respectively, were used for high heat flux characterizations. A liquid-cooled aluminum heat sink with an area of 15 mm (L) /spl times/12.2 mm (W) populated by microchannels was designed and fabricated. The microchannel heat sink was assembled onto the chip, using a thermal interface material to reduce the contact thermal resistance at the interface. A variable speed pump was used to provide the pressure head for the liquid cooling loop. The measured thermal resistance results ranged from 0.44 to 0.32/spl deg/C/W for the 12-mm chip case and from 0.59 to 0.44/spl deg/C/W for the 10-mm chip case, both under flowrates ranging from 1.67/spl times/10/sup -6/ m/sup 3//s to 1.67/spl times/10/sup -5/ m/sup 3//s. An analytical model of the flow and heat transfer in microchannel heat sinks is also presented. Computational predictions agree with the measurements for pressure drop within 15% and thermal resistances within 6%. The analytical results indicate that thermal interface resistance becomes a key limitation to maximizing heat removal rate from electronic packages.     

Effect of the doping level of a p-cladding layer on the performance of GaAs-AlGaAs multiquantum-well lasers

CW performance of multiquantum-well (MQW) GaAs-AlGaAs lasers operating near 850 nm at 300 K is investigated as a function of the doping level in the p-cladding layer. Laser structures have been grown by metal-organic chemical vapor deposition using Zn diethyl as a carrier for acceptor dopant. An undoped sublayer (setback) has been introduced to separate the heavily doped part of the cladding from the mode volume. The doping level of zinc ranges from 4/spl times/10/sup 17/ to 6.5/spl times/10/sup 18/cm/sup -3/. Along with this increase of the doping level, the threshold current density increases from 370 to 612 A/cm/sup 2/ whereas the slope efficiency of the laser increases from 0.71 to 1.33 W/A with an improvement of the characteristic temperature constant T/sub 0/ from 150 to 250 K. At the highest level of doping, the internal quantum yield is found to be close to unity and optical losses are as low as 1.36 cm/sup -1/. An increase of the stable single-mode output is also obtained in 3-/spl mu/m-wide ridge-waveguide diodes up to /spl sim/180 mW.     

An ultrathin vertical channel MOSFET for sub-100-nm applications

An ultrathin vertical channel (UTVC) MOSFET with an asymmetric gate-overlapped low-doped drain (LDD) is experimentally demonstrated. In the structure, the UTVC (15 nm) was obtained using the cost-effective solid phase epitaxy, and the boron-doped poly-Si/sub 0.5/Ge/sub 0.5/ gate was adopted to adjust the threshold voltage. The fabricated NMOSFET offers high-current drive due to the lightly doped (<1/spl times/10/sup 15/ cm/sup -3/) channel, which suppresses the electron mobility degradation. Moreover, an asymmetric gate-overlapped LDD was used to suppress the offstate leakage current and reduce the source/drain series resistance significantly as compared to the conventional symmetrical LDD. The on-current drive, offstate leakage current, subthreshold slope, and DIBL for the fabricated 50-nm devices are 325 /spl mu/A//spl mu/m, 8/spl times/10/sup -9/ /spl mu/A//spl mu/m, 87 mV/V, and 95 mV/dec, respectively.     

Normal-incidence InAs self-assembled quantum-dot infrared photodetectors with a high detectivity

An InAs/AlGaAs quantum-dot infrared photodetector based on bound-to-bound intraband transitions in undoped InAs quantum dots is reported. AlGaAs blocking layers were employed to achieve low dark current. The photoresponse peaked at 6.2 /spl mu/m. At 77 K and -0.7 V bias, the responsivity was 14 mA/W and the detectivity, D*, was 10/sup 10/ cm/spl middot/Hz/sup 1/2//W.     

Effect of doped substrate on GaAs-AlGaAs interfacial workfunction IR detector response through cavity effect

In this paper, results are reported showing response enhancement in GaAs-AlGaAs IR detectors using a doped substrate to increase reflection, enhancing the resonant cavity effect. Responsivity for heterojunction interfacial workfunction detectors grown on semi-insulating (SI) and doped substrates are compared. For a device grown on an SI substrate, a 9-/spl mu/m resonance peak had a response of 1.5 mA/W while a similar device on an n-doped substrate showed 12 mA/W. Also, the difference between response under forward and reverse bias (3 versus 12 mA/W) for the sample grown on the doped substrate, as well as calculated results confirm that the increased response is due to the resonant enhancement. An optimized design for a 15-/spl mu/m peak (24 /spl mu/m 0 response threshold) detector grown on a doped substrate could expect a peak response of 4 A/W with a 50% quantum efficiency and D/sup */ /spl sim/ 2 /spl times/ 10/sup 10/ Jones at the background limited temperature of 50 K.     

Low-frequency noise characteristics of p-n-p InAlAs/InGaAs HBTs

The low-frequency noise characteristics of p-n-p InAlAs/InGaAs heterojunction bipolar transistors (HBTs) were investigated. Devices with various geometries were measured under different bias conditions. The base noise current spectral density (3.11 /spl times/ 10/sup -16/ A/sup 2//Hz) was found to be higher than the collector noise current spectral density (1.48 /spl times/ 10/sup -16/ A/sup 2//Hz) at 10 Hz under low bias condition (I/sub C/=1 mA, V/sub EC/=1 V), while the base noise current spectral density (2.04 /spl times/ 10/sup -15/ A/sup 2//Hz) is lower than the collector noise current spectral density (7.87 /spl times/ 10/sup -15/ A/sup 2//Hz) under high bias condition (I/sub C/=10 mA, V/sub EC/=2 V). The low-frequency noise sources were identified using the emitter-feedback technique. The results suggest that the low-frequency noise is a surface-related process. In addition, the dominant noise sources varied with bias levels.     

Self-phase modulation coefficient of multiple-quantum-well optical amplifiers

The self-phase modulation coefficient /spl gamma/ of 1310 nm multiple-quantum-well (MQW) semiconductor optical amplifiers has been investigated. It is found to vary from 16/spl times/10/sup 4/ W/sup -1/ m/sup -1/ for low driving conditions to 3/spl times/10/sup 4/ W/sup -1/ m/sup -1/ for high-driving conditions. This implies that the amount of self-phase modulation occurring in the amplifier is between 1.5-10/spl times/ more than that occurring in the optical fiber following the amplifier. The additional self-phase modulation caused by the semiconductor optical amplifier may be used to achieve compensation for fiber dispersion in optical communication systems at significantly lower average power levels. The linewidth enhancement factor /spl alpha//sub H/ was found to increase from a value of 2 at low driving conditions, in agreement with results reported for MQW lasers, to a value of 3 at high-driving conditions.     

Nonalloyed ohmic contacts in AlGaN/GaN HEMTs by ion implantation with reduced activation annealing temperature

This letter reports AlGaN/GaN high-electron mobility transistors with capless activation annealing of implanted Si for nonalloyed ohmic contacts. Source and drain areas were implanted with an Si dose of 1/spl times/10/sup 16/ cm/sup -2/ and were activated at /spl sim/1260/spl deg/C in a metal-organic chemical vapor deposition system in ammonia and nitrogen at atmospheric pressure. Nonalloyed ohmic contacts to ion-implanted devices showed a contact resistance of 0.96 /spl Omega//spl middot/mm to the channel. An output power density of 5 W/mm was measured at 4 GHz, with 58% power-added efficiency and a gain of 11.7 dB at a drain bias of 30 V.     

High mobility NMOSFET structure with high-/spl kappa/ dielectric

High-/spl kappa/ NMOSFET structures designed for enhancement mode operation have been fabricated with mobilities exceeding 6000 cm/sup 2//Vs. The NMOSFET structures which have been grown by molecular beam epitaxy on 3-in semi-insulating GaAs substrate comprise a 10 nm strained InGaAs channel layer and a high-/spl kappa/ dielectric layer (/spl kappa//spl cong/20). Electron mobilities of >6000 and 3822 cm/sup 2//Vs have been measured for sheet carrier concentrations n/sub s/ of 2-3/spl times/10/sup 12/ and /spl cong/5.85/spl times/10/sup 12/ cm/sup -2/, respectively. Sheet resistivities as low as 280 /spl Omega//sq. have been obtained.     

Fully silicided NiSi gate on La/sub 2/O/sub 3/ MOSFETs

We have fabricated the fully silicided NiSi on La/sub 2/O/sub 3/ for n- and p-MOSFETs. For 900/spl deg/C fully silicided CoSi/sub 2/ on La/sub 2/O/sub 3/ gate dielectric with 1.5 nm EOT, the gate dielectric has large leakage current by possible excess Co diffusion at high silicidation temperature. In sharp contrast, very low gate leakage current density of 2/spl times/10/sup -4/ A/cm/sup 2/ at 1 V is measured for 400/spl deg/C formed fully silicided NiSi and comparable with Al gate. The extracted work function of NiSi was 4.42 eV, and the corresponding threshold voltages are 0.12 and -0.70 V for respective n- and p-MOSFETs. Electron and hole mobilities of 156 and 44 cm/sup 2//V-s are obtained for respective n- and p-MOSFETs, which are comparable with the HfO/sub 2/ MOSFETs without using H/sub 2/ annealing.