L. Davis

Carrier capture and relaxation in narrow quantum wells

In separate confined heterostructure (SCH) lasers, injected electrons and holes thermalize into a quantum well after diffusion through the outer cladding layers. The carriers move towards equilibrium by emitting optical phonons. In narrow quantum wells, as compared to the 1-2 ps required in bulk semiconductors, this phonon emission process can be considerably slowed down due to the 2-D density of states and the nature of the electron-optical phonon interaction. This process has been studied theoretically using a Monte Carlo program which allows us to see the carrier distribution as a function of time. Typical times for carrier relaxation are 10-15 ps for a 50 /spl Aring/ GaAs well with Al/sub 0.30/Ga/sub 0.70/As barriers and /spl sim/5 pS for a 200 /spl Aring/ well. These calculations have been complemented by time-resolved photoluminescence measurements on SCH structures where the relaxation time from a 3D distribution into In/sub 0.20/Ga/sub 0.80/As/GaAs wells is measured at T=200 K. Carrier relaxation times of 50, 41, 22, and 17 ps are obtained for wells of sizes 30, 40, 50, and 100 /spl Aring/, respectively. The results show clearly that the use of narrow quantum wells in low threshold lasers will pose a serious limitation to the efficiency and small-signal modulation bandwidth of these devices. >

Inner ear anomalies in Waardenburg's syndrome associated with Hirschsprung's disease.

Congenital inner ear anomalies are reported in temporal bones of a 22-month-old boy with Waardenburgs syndrome and Hirschsprungs disease. Although no changes in the central auditory pathway were identified, peripheral lesions of the cochlear and vestibular membranous labyrinth were observed. Bilateral atrophy of the organ of Corti and stria, and a sparsity of spiral ganglion cells were observed in the cochlea. Degeneration of the vestibular end organs, including a loss of Scarpas ganglion cells, was also seen. This is the first report of temporal bone histopathology associated with Waardenburgs syndrome and Hirschsprungs disease. The pathoembryology of these inner ear anomalies associated with aganglionosis of the colon supports the hypothesis that Waardenburgs syndrome and Hirschsprungs disease are hereditary defects of neural crest cells.

Small‐signal modulation and temperature dependence of the tunneling injection laser

Recently, we demonstrated a novel laser structure, called the tunneling injection laser, where the electrons are injected into the active lasing quantum well region via tunneling. High performance results for this device have now been demonstrated. A To of 160 K was found from temperature‐dependent measurements (25–70 °C). High differential gain (5.5×10−16 cm2) and modulation bandwidth (12.5 GHz) have been attained relative to other single quantum well lasers.

Auger recombination rates in compressively strained In/sub x/Ga/sub 1-x/As/InGaAsP/InP (0.53<or=x<or=0.73) multiquantum well lasers

The authors have experimentally determined Auger recombination rates in compressively strained In/sub x/Ga/sub 1-x/As/InGaAsP/InP MQW lasers for the first time. The Auger recombination rates were derived from the measured turn-on delay times during large-signal modulation of single-mode lasers. The Auger coefficient increases from 5+or-1*10/sup -30/ to 13+or-1*10/sup -30/ cm/sup 6/ s/sup -1/ as the indium composition in the quantum well active region, x, increases from 0.53 to 0.73.<<ETX>>

The growth of resonant tunneling hot electron transistors using chemical beam epitaxy

A systematic growth study of InGaAs/AIA5/InGaAsP resonant tunneling hot electron transistors (RHETs) was performed using chemical beam epitaxy (CBE). The resonant tunneling hot electron transistors studied consist of a highly strained AIAs/1n075Ga525As/AIA5 double barrier structure and an undoped InP collector barrier with 1.1 and 1.2 gm InGaAsP graded layers. These quaternaries were lattice matched to InP within 2.6x iO~ and showed an averaged full width at half-maximum (FWHM) of 6 meV from low temperature photoluminescence (PL) measurement. The effects of growth interrupt were studied using PL, X-ray diffraction and secondary ion mass spectrometry (SIMS) measurements, It was found that excessive growth interrupt induced high oxygen accumulation (8x io’~ cm 3) at the heterojunction and reduced the intensity of PL spectra. Moreover, for the growth of tunneling heterostructures, low substrate temperature, appropriate growth interrupts and use of hydride drying filters and high purity hydrides were helpful to improve device performance. The highest peak-to-valley current ratio (PVR) observed was 12.7, and maximum base transport ratio was 0.98 at 80 K. Furthermore, some digital functions such as flip-flop gate and exclusive NOR were demonstrated using a single RHET.

Influence of hydride purity on InP and InAlAs grown by chemical beam epitaxy

It has generally been recognized that sources of the highest purity facilitate growth of loP and InAlAs with excellent optical and electrical characteristics. The mobility, photoluminescence linewidths, X-ray linewidths, impurity levels, and ultimately device results have been evaluated for two different hydride purity levels. It was found that each of the above characteristics was affected by improved arsine and phosphine purity. The 77 K mobility of lnP increased by a factor of 2 and the InAlAs silicon doping efficiency increased by a factor of 5. Accompanying improvement in device performance, notably the peak to valley ratio of resonant tunneling structures has been observed.

Observation of multiple resonance frequencies in stripe geometry InGaAs/InGaAsP/InP quantum-well lasers

Intensity fluctuation noise in strained InGaAsP/InP multi-quantum-well lasers is analyzed for both ridge-guided and broad-area gain-guided structures. A single resonance peak is observed in the noise spectrum for the ridge-guided laser, as expected. However, the noise spectrum for the broad-area lasers shows multiple ( approximately 2-5) resonance peaks, distinctly spaced, from approximately 2 to approximately 5 GHz. Combined with near-field measurements, the experiments show that these peaks originate from lasing filaments having significantly nonuniform optical power. The authors also determined the resonance frequency of the single-mode laser from both small-signal modulation and turn-on relaxation oscillation measurements and found the results to be consistent with the measured peak noise frequency. >

Lasers for large modulation bandwidth and low-threshold applications

Carrier transport and recombination dynamics are seen to be the intrinsic limitations to the performance of quantum well lasers. The carrier relaxation times as a function of quantum well width were measured in laser structures using a streak camera. Auger recombination rates were experimentally determined in compressively strained InxGa1-xAs/InGaAsP/InP quantum wells from the large signal modulation of single mode lasers. In order to overcome the intrinsic limitations in present semiconductor laser designs, a new device concept has been demonstrated: the tunneling injection quantum well laser, in which the carriers are injected into the active lasing subband by resonant and sequential tunneling. The highest 3 dB modulation bandwidth (12.5 GHz) and the highest differential gain (6 X 10-16 cm2) for a single quantum well laser have already been demonstrated. To realize threshold currents of much less than 1 mA, quantum wire lasers are required. We present theoretical and experimental results on the performance characteristics of quantum wire lasers. The experimental structures are being realized in the InxGa1-xAs/GaAs system by MBE growth and regrowth and electron beam lithography.

Modulation characteristics of InP-based MQW lasers: the impact of biaxial compressive and tensile strain

Summary form only given. The authors relate observed modulation characteristics of strained In/sub x/Ga/sub 1-x/As-InP (0.33 >