Reach-through Avalanche Photodiodes Research Articles

Novel sensor developments for photon science at the MPG semiconductor laboratory

The world of photon science experiences significant advancements since the advent of synchrotron light sources with unprecedented brilliance, intensity and pulse repetition rates, with large implications on the detectors used for instrumentation. Here, an overview about the work on this field carried out at the semiconductor laboratory of the Max-Planck-Society (MPG HLL) is given. Main challenges are high dynamic range to resolve faint features at the fringes of scatter images as well as structures in bright peaks, and high bandwidth to fully exploit the fast timing capability of the source. A newly developed device to improve the signal-to-noise-ratio (SNR) at high bandwidths is the so-called MARTHA (Monolithic Array of Reach-Through Avalanche Photodiodes) structure, which integrates an array of APDs on a monolithic substrate. The reach-through architecture assures near 100% fill factor and allows implementing a thin entrance window with optimized quantum efficiency for low energy X-rays. The structures operate in proportional mode with adjustable gain, and can serve as a drop-in replacement for PAD detectors in hybrid pixel systems. A more sophisticated solution for low to medium frame rate applications with high contrast requirement are pnCCDs with high dynamic range in the pixel area featuring DEPFET based readout nodes with non-linear amplification (NLA). The high dynamic range mode has been demonstrated for pnCCD devices with a pixel size down to 75 μm2. Framerates of up to 1 kHz are possible for a 1 Megapixel detector. Small size prototypes of these structures have recently been manufactured. Modified DEPFET structures with build-in non-linear amplification are also used to implement active pixel detectors optimized for high dynamic range. Successfully prototyped for the DSSC sensors (DEPFET Sensor with Signal Compression) at the XFEL, these structures are increasingly being used in applications requiring high contrast and intensity, e.g., TEM imaging. Charge handling capability and output characteristics can be tailored to the requirements, as well as pixel geometry and size. The large intrinsic gain of the DEPFET provides excellent SNR even at fast timing. Pixels can be read with a speed of 100 ns, the resulting frame rate depends on the degree of readout parallelization.

APD Rise Time Measurements for 50–300-keV Ions

In this paper, we report the rise time of three silicon reach-through avalanche photodiodes (APD) when measuring alpha particles and low-energy ions in the energy range from 50 to 300 keV. The APDs were custom manufactured by Hamamatsu Photonics and differ by their doping manufacturing method (p-type or n-type) and their depletion layer thickness (30 or $150~\mu \text{m}$ ). Our findings are summarized as follows: 1) the rise time is largely a function of detector drift layer thickness and 2) the rise time is mostly independent of ion species. The measured rise time values were comparable to ~1.5 ns for the two $30~\mu \text{m}$ APDs at the operational bias voltages. In combination with the timing electronics, such as constant fraction discriminators, a less than 1-ns response time resolution will be achievable for these APDs. These new findings are valuable for applying APDs for suprathermal particle measurements in space physics, enabling simplified time-of-flight and energy systems, and high count rate applications.

Improvement of CMOS-Integrated Vertical APDs by Applying Lateral Well Modulation

A method for the improvement of CMOS-compatible reach-through avalanche photodiodes is presented, which works without any process modification. Special fill patterns are used in order to achieve an arbitrary modulation doping factor of the used well. Due to lateral diffusion, the dopant concentration is smoothed and a well results which is in average less doped than the original well. This gives additional degrees of freedom for the design of CMOS-integrated APDs without needing process modifications. The method is verified by the characterization of six avalanche photodiodes with different modulation doping factors for the multiplication zone fabricated in a 0.35 $\mu \text{m}$ CMOS process. The breakdown voltage is adjusted within the range from 15.4 to 182 V while the property of avalanche multiplication of photo-generated carriers is kept. The CMOS process used allowed the isolation of APDs and circuits on the same chip for modulation doping factors between 100% and 50%, which resulted in an increase of the APDs bandwidth from 8 MHz (100% doping) to 386 MHz (50% doping).

Energy Resolution Contributions in Reach-Through Avalanche Photodiodes

Avalanche photodiodes (APD) are used in the muonic helium Lamb shift experiment for detection of 8 keV X-rays. Reach-through APDs from Hamamatsu Photonics have been investigated for X-ray detection as alternative to conventional APDs. In order to evaluate the different contributions to the energy resolution obtained in a reach-through APD, its response to both 8 keV X-rays and visible light pulses was investigated. The intrinsic resolution was estimated by further measurements of the electronic noise contribution and the gain non-uniformity was assessed. The excess noise factor was determined. Results at different temperatures are presented.

Reach-Through Avalanche Photodiodes in Soft X-ray Detection

Reach-through avalanche photodiodes (RT-APDs) from Hamamatsu Photonics, with different active areas, 5 × 5 and 3 × 3 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , were investigated to evaluate their suitability for the muonic helium Lamb shift experiment. The gain has been determined as a function of voltage and temperature for both prototypes. As expected, it increases with increasing bias voltage and with decreasing temperature. The gain variation with temperature is smaller than per -2% °C even for higher bias voltages applied to the RT-APD. The non-linearity between gain obtained for X-rays and visible light pulses has been investigated for different temperatures. The non-linearity was found to increase with decreasing temperature. For example at 350 V, it is as high as 25% at -20 °C and about 10% at 20 °C. The RT-APDs performance for the detection of 8 keV X-rays has been investigated. The best energy resolution was achieved for the larger prototype for a temperature of 0 °C. The minimum energy resolution values, between 9.2 and 9.9%, were obtained for gains between 55 and 80. The minimum detectable energy observed is about 0.2 keV for the higher gain region, for both RT-APDs, investigated, with no significant improvement at lower temperatures.

The Effect of Electron versus Hole Photocurrent on Optoelectric Properties of p+-p-n-n+ Wz-GaN Reach-Through Avalanche Photodiodes

The authors have made an attempt to investigate the effect of electron versus hole photocurrent on the optoelectric properties of p+-p-n-n+ structured Wurtzite-GaN (Wz-GaN) reach-through avalanche photodiodes (RAPDs). The photo responsivity and optical gain of the devices are obtained within the wavelength range of 300 to 450 nm using a novel modeling and simulation technique developed by the authors. Two optical illumination configurations of the device such as Top Mounted (TM) and Flip Chip (FC) are considered for the present study to investigate the optoelectric performance of the device separately due to electron dominated and hole dominated photocurrents, respectively, in the visible-blind ultraviolet (UV) spectrum. The results show that the peak unity gain responsivity and corresponding optical gain of the device are 555.78 mA W−1 and 9.4144×103, respectively, due to hole dominated photocurrent (i.e., in FC structure); while those are 480.56 mA W−1 and 7.8800×103, respectively, due to electron dominated photocurrent (i.e., in TM structure) at the wavelength of 365 nm and for applied reverse bias of 85 V. Thus, better optoelectric performance of Wz-GaN RAPDs can be achieved when the photocurrent is made hole dominated by allowing the UV light to be shined on the n+-layer instead of p+-layer of the device.

High quantum efficiency GaP avalanche photodiodes

Gallium Phosphide (GaP) reach-through avalanche photodiodes (APDs) are reported. The APDs exhibited dark current less than a pico-ampere at unity gain. A quantum efficiency of 70% was achieved with a recessed window structure; this is almost two times higher than previous work.

Drift-diffusion model for reach-through avalanche photodiodes

A 1-D drift-diffusion model is developed for reach-through avalanche photodiodes. Based on this model an equivalent circuit is suggested for these devices. The equivalent circuit is suitable for simulating the device in circuit simulation packages such as PSpice. The model is validated by simulating an actual reach-through silicon avalanche photodiode in the steady state and the transient conditions. Some of the simulation results are compared to the manufacturer data by estimating the device parameters. This model is also used to investigate the effect of the device microstructure on its performance. For a certain operating bias voltage and wavelength, it is found that the multiplication width has a critical value for defining the device stability. The absorption width defines the wavelength for peak responsivity, which suggests a structure enhancement for IR detection. Also the effect of varying the doping concentration is studied.

Demonstration of the first 4H-SiC avalanche photodiodes

4H-SiC visible-blind reach-through avalanche photodiodes (RAPDs) were designed and fabricated with mesa edge termination and thermal oxide passivation techniques. The devices show “hard” avalanche breakdown with a positive temperature coefficient. The photo response spectra, measured at different biases, displayed a maximum responsivity of 106 A/W, and a corresponding optical gain of about 500.

Application of avalanche photodiodes as a readout for scintillator tile–fiber systems

The application of reach-through avalanche photodiodes (R'APD) as a photodetector for scintillator tiles has been investigated. The light collected by WLS fibers (0.84 and 1 mm diameter) embedded in the scintillator has been transmitted to the 0.5 mm 2 active surface of APD by clear optical fibers and optical connectors. A low noise charge sensitive preamplifier (≈400e − equivalent noise charge) has been used to amplify the photodiode signal. Various configurations of tile–fiber systems, suitable for CMS and LHCb experiments at LHC have been studied using cosmic muons and muon beam at SPS at CERN. In order to optimize the performance of APD, measurements in the temperature range from −10°C to +25°C have been made. The MIP detection efficiency and e −/MIP separation have been estimated in order to determine applicability of the readout for LHCb preshower.

Application of avalanche photodiodes as a readout for scintillator tile-fiber systems

The application of reach-through avalanche photodiodes (R'APD) as a photodetector for scintillator tiles has been investigated. The light collected by WLS fibers (0.84mm and 1mm diameter) embedded in the scintillator has been transmited to the 0 .5mm2 active surface of APD by clear optical fibers and optical connectors. A low noise charge sensitive preamplifier (approximately 400 electrons equivalent noise charge) has been used to gain the photodiode signal. Various configurations of tile-fibre s ystems, suitable for CMS and LHCb experiments at LHC have been studied using cosmic muons and muon beam at SPS at CERN. In order to optimize the performance of APD, measurments in the temperature range from -10°C to +25°C have been done. The MIP detection efficiency and electron/MIP separation have been estimated in order to determine applicability of the readout for LHCb preshower.

Recent detector developments at SINTEF (industrial presentation)

Results from SINTEF's research on radiation hardness of silicon detectors, thin silicon detectors, silicon drift devices, reach-through avalanche photodiodes, and detectors with thin dead layers are presented.

A fast, convenient, X-ray detector

We investigate the response of 5 × 5 mm 2 silicon reach-through avalanche photodiodes followed by high bandwidth electronics to X-rays between 6 and 30 keV. In particular, we demonstrate high-dynamic range (∼10 9) with maximum count rates in excess of 10 7 Hz, and good, ∼20%, pulse-height resolution with the same amplification scheme. Single diodes have good efficiencies below 20 keV while a stack of diodes is shown to have better than 30% efficiency at 30 keV. The time resolution of these devices is about 1.6 ns. Application to nuclear scattering experiments with synchrotron radiation is discussed.

Electrical performance of laser damaged silicon photodiodes

Laser induced electrical parameter degradation and morphological damage have been observed in silicon photodiodes. The samples were RCA reach-through avalanche photodiodes and EG&G PIN photodiodes. The laser source was a 1064-nm Q-switched Nd: YAG laser (10-Hz, 10-ns pulses with a 300-microm spot radius). Reverse saturation current, noise current, breakdown voltage, junction capacitance, and surface morphology were monitored for permanent laser induced change. The current characteristics were clearly the most sensitive electrical parameters; however, the electrical performance was generally insensitive to severe surface morphological damage. The damage behavior indicated that the electrical degradation in photodiodes may be modeled by the introduction of defects into the depletion region by deep melting transients.

Electron and hole impact ionization coefficients in (100) and in (111) Si

The impact ionization coefficients for electrons and holes in both (100)- and (111)-oriented Si have been determined from photocurrent measurements performed on reach-through avalanche photodiodes. The results for both orientations are similar, indicating that the ionization coefficients are isotropic. Monte Carlo calculations of the impact ionization coefficients compare favorably with the experimental results.

Recent developments in silicon avalanche photodiodes

Recent developments in silicon reach-through avalanche photodiodes (RAPDs) are reviewed. These include devices with: •Broader spectral response (quantum efficiency > 60% from 400 to 1000nm)•Lower bulk dark-current densities•Lower excess-noise-factor devices (keff reduced from .02 to .006)•Multi-element APDs. . These developments make APDs suitable for a number of new applications, two of which are discussed in some detail. They are: 1.(1) RAPDs used for single-photn counting in bith the normal (V < VB) and ‘Geiger’ (V > VB) modes. The adavantages adn diasadvantages of each of these modes of operation are discussed.2.(2) Large-area RAPDs used with scintillators for gamma ray detection. This development is of particular interest for high-resolution positron-emission tomography (PET) medical imaging systems.

Multi-element reachthrough avalanche photodiodes

Two approaches to making multi-element arrays of p+-π-p-n+reachthrough avalanche photodiodes are reported. In the first approach a single common avalanche region (p-layer) for all elements is used, with the segmentation between elements being on the p+layer. This approach has the advantage of having zero dead space between adjacent elements, but is difficult to fabricate, and has a very narrow range of operation in which it is neither noisy due to injection nor suffers from poor element-to-element isolation. In a second approach, the p+contact is common and separate avalanche regions are used. The problem for this case is the width of the dead space between adjacent elements which, because of field-fringing effects, is considerably wider than the actual physical distance between elements. A self-aligning technique is described for fabricating arrays by the second approach and the technique demonstrated with a 25-element linear array on 300-µm centers. The measured dead space is in the 60-80 µm range, depending on the gain. The array can be used at an average gain of 100 or more, has excellent element-to-element isolation, and NEP's below 2 × 1015W/Hz1/2at 800-900 nm and below 10-14W/ Hz1/2over the whole spectral range from 400 to 1060 nm.

Germanium reachthrough avalanche photodiodes for optical communication systems at 1.55-µm wavelength region

Germanium reachthrough avalanche photodiodes (Ge RAPD's) with high-frequency response have been designed, fabricated, and tested. In the calculation of frequency response, optimum depletion layer width of 21 µm has been found for 1.55-µm wavelength with the highest cutoff frequency of 830 MHz. The diodes fabricated by this design showed frequency degradation of less than 2 dB at 500 MHz and at 1.55 µm. This response has been unchanged up to 1.58 µm, indicating useful spectral limit lies at more than 1.58 µm. The diodes exhibited quantum efficiency of 80 percent and excess noise factor of 6.1 at a multiplication of 10 both for 1.55 µm. The breakdown voltage was 60- 90 v. The sensitivity of the diodes was measured at 100 Mb/s and 1.55 µm. The minimum detectable power of -44.3 dBm which is by 5.2 dB better than the conventional p <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> -n Ge APD has been achieved for 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-11</sup> error rate. Comparison with InGaAs APD and p-i-n/FET receiver has been made by calculating minimum detectable power of RAPD at 500 Mb/s. Calculated sensitivity of RAPD is 1-2 dB worse than InGaAS APD and comparable to that of InGaAs p-i-n/FET receiver estimated from the reported experimental results.

Noise, responsivity and prospects for long-term stability of silicon reach-through avalanche photodiodes

Approximately 100 silicon reach-through avalanche photo-diodes of three different types have been characterised for the dependence of noise and responsivity on bias voltage, to indicate margins to be allowed for variations between devices in optical systems operating at 0.85 μm wavelength. Long-term stability measurements reveal a correlation between reliability and production batch.

Wavelength and temperature dependence of RAPD laser detectors

Wavelength and temperature-dependent characteristics of silicon reach-through avalanche photodiodes (RAPD) are studied using the modified Baraff's theory. The temperature and wavelength regions discussed are 100-500 K and 0.655-1.06 microm, respectively, for Si photodetectors. The effects of breakdown voltage, depletion region width, and efficiency on the avalanche region width are widely studied. Both n(+) (front) side and p(+) (back) side illumination are also considered. The noise equivalent power is significantly reduced with light incident on the p(+) surface with little sacrifice of efficiency. An RAPD structure is proposed in which the direction of radiation incidence causes photogeneration to precede avalanche multiplication reducing the multiplication noise substantially. This result should be useful in the design of RAPD laser detectors.