P.N. Robson

A Monte Carlo investigation of multiplication noise in thin p/sup +/-i-n/sup +/ GaAs avalanche photodiodes

A Monte Carlo (MC) model has been used to estimate the excess noise factor in thin p/sup +/-i-n/sup +/ GaAs avalanche photodiodes (APDs). Multiplication initiated both by pure electron and hole injection is studied for different lengths of multiplication region and for a range of electric fields. In each ease a reduction in excess noise factor is observed as the multiplication length decreases, in good agreement with recent experimental measurements. This low noise behavior results from the higher operating electric field needed in short devices, which causes the probability distribution function for both electron and hole ionization path lengths to change from the conventionally assumed exponential shape and to exhibit a strong dead space effect. In turn this reduces the probability of higher order ionization events and narrows the probability distribution for multiplication. In addition, our simulations suggest that fur a given overall multiplication, electron initiated multiplication in short devices has inherently reduced noise, despite the higher feedback from hole ionization, compared to long devices.

Investigation of impact ionization in thin GaAs diodes

The electron and hole multiplication coefficients, M/sub e/ and M/sub h/, respectively, have been measured in thin GaAs homojunction PIN and NIP diodes and from conventional ionization analysis the effective electron and hole ionization coefficients, /spl alpha/ and /spl beta/, respectively, have been determined. The nominal intrinsic region thickness w of these structures ranges from 1.0 /spl mu/m down to 25 nm. In the thicker structures, bulk-like behavior is observed; however, in the thinner structures, significant differences are found. As the i-regions become thinner and the electric fields increase, the M/sub e//M/sub h/ ratio is seen to approach unity. The experimental results are modeled and interpreted using a semianalytical solution of the Boltzmann equation. In thin (w/spl les/0.1 /spl mu/m) devices the dead space effect reduces effective ionization coefficients below their bulk values at low values of carrier multiplication. However, overshoot effects compensate for this at extremely high fields (/spl ges/1/spl times/10/sup 3/ kV/cm).

A simple model to determine multiplication and noise in avalanche photodiodes

Avalanche multiplication and noise in 1.0, 0.5, 0.1, and 0.05 μm GaAs p+-i-n+ diodes have been calculated for both electron and hole initiated multiplication using a simple model which incorporates a randomly-generated ionization path length (RPL) and a hard-threshold dead space. We find that the mean multiplication obtained using this RPL model is in excellent agreement, even for the shortest structure, with that obtained from an analytical-band structure Monte Carlo (MC) model, which incorporates soft-threshold effects. However, it predicts slightly lower avalanche noise in the shorter devices. This difference results from the narrower ionization path length probability distribution and larger dead space of the hard-threshold RPL model at high electric fields as compared to the more realistic distribution function associated with the relatively sophisticated MC model.

InP Schottky contacts with increased barrier height

We present an analysis of Schottky barriers in n-InP made by incorporating a thin native oxide. An oxidation technique using nitric acid under illumination produces an oxide layer with uniform composition distribution within the layer. The growth rate is interpreted as being partially limited by diffusion presumably of oxygen through oxide. The Au Schottky barrier formed on a 40–80 A thick oxide layer exhibits little degradation of the ideality factor n (1.04 < n < 1.10) and an increase of the barrier height by greater than 0.3 eV, resulting in at least a 10−4 times smaller reverse leakage current density, compared with conventional Au-InP barriers. The barrier height increase is analysed by a generalised model, and is found to be produced by the existence of fixed negative charges in the oxide layer. From the present analysis, a surface state density of 6.0 × 1012 cm−2 eV−1 and an equivalent surface density of negative charges of 2.8 × 1012 cm−2 are determined independently. The origins of these, particularly of the surface states, are considered in relation to the P vacancies at the oxide-InP interface.

Tailoring of internal fields in InGaAs/GaAs multiwell structures grown on (111)B GaAs

We present a study of internal field distributions in strained InGaAs/GaAs multiple quantum wells in p‐i‐n structures grown on (111)B‐oriented GaAs. Room temperature photocurrent spectroscopy shows clear blueshifting of the e1‐hh1 transition as the well fields are reduced by external bias. The relative length of total well to total barrier material is shown to be an important factor in determining the well and barrier fields. We demonstrate a photocurrent contrast ratio of 4.5:1 for only 3 V applied bias across a 25 quantum well In0.13Ga0.87As p‐i‐n diode and discuss the implication of our results to the design of high performance electro‐optic modulators and self electro‐optic effect devices in this material system.

Gain of a heterojunction bipolar phototransistor

Analytical expressions have been derived for the collector current, optical gain, and quantum efficiency for a heterojunction, bi-polar phototransistor (HPT). These expressions can be utilized to optimize the current gain and quantum efficiency for HPT design. The presence of avalanche multiplication in the base-collector junction has been taken into account and shown to be a significant factor in determining the gain of an InGaAs/InP phototransistor. Experimental results of optical gain versus the collector-emitter voltage can only be explained in terms Of avalanche multiplication.

Diffusion and the power spectral density of velocity fluctuations for electrons in InP by Monte Carlo methods

The low‐frequency parallel and transverse diffusion coefficients for electrons in InP are calculated using Monte Carlo methods for lattice temperatures of 200, 300, and 500 K. The sensitivity of the computed values to the duration of the flight sampling time is investigated, and this is shown to be a consequence of the frequency dependence of the diffusion coefficient. To test this hypothesis the variation of the parallel diffusion coefficient with both frequency and field at 300 K is determined. For fields above threshold the parallel diffusion coefficient first rises from its low‐frequency value to a maximum and thereafter decreases with increasing frequency; for fields substantially below threshold a slow monotonic decrease is observed.

Full band Monte Carlo modeling of impact ionization, avalanche multiplication, and noise in submicron GaAs p+-i-n+ diodes

A full-band Monte Carlo model is used to investigate the probability distribution functions of impact ionization path length and impact ionization energy for electrons and holes in GaAs. The simulations show that the soft ionization threshold energy in GaAs allows impact ionization to occur at energies much higher than the band gap. As a result, secondary carriers have a shorter dead space than newly injected carriers. The ionization path length distributions narrow at higher fields, producing a more deterministic impact ionization process in thin devices. The model is also used to simulate avalanche multiplication and noise in submicron homojunction GaAs p+-i-n+ diodes. The predicted mean multiplication, 〈M〉 and excess noise factor, F are in quantitative agreement with the experimental results, in which F decreases as the length of multiplication region is reduced.

130 PS RECOVERY OF ALL-OPTICAL SWITCHING IN A GAAS MULTIQUANTUM WELL DIRECTIONAL COUPLER

A significantly improved recovery time of 130 ps has been achieved in an all‐optical zero‐gap directional coupler containing multiple quantum wells. The mechanism for the all‐optical switching is due to free‐carrier induced refractive nonlinearities at near‐band‐gap resonant frequencies. The large reduction in the switch recovery time was obtained by the application of an external dc bias which sweeps out the carriers from the quantum wells. From our experimental results on the laser pulse width limited switch‐up time of 2 ps, we deduce that the contribution of self‐electro‐optic effect nonlinearity is negligible in this case.

New formula for semiconductor laser facet reflectivity

An analytic expression for the main mode reflectivity of a multicoated laser facet at normal incidence is derived using the free space radiation mode technique. This uses exact guided modes, and accurately models the radiation modes near the facet. The results compare favorably with exact TE and TM benchmarks while retaining the simplicity and speed of the Fresnel approximations.<<ETX>>