Anne Lorenz

On the blistering of atomic layer deposited Al 2 O 3 as Si surface passivation

This work proves that blistering is the partial delamination of a thick enough Al<inf>2</inf>O<inf>3</inf> layer caused by gaseous desorption in the Al<inf>2</inf>O<inf>3</inf> layer upon thermal treatments above a critical temperature: the Al<inf>2</inf>O<inf>3</inf> layer acts as a gas barrier and bubble formation occurs. First, using an atmospheric pressure rapid thermal processor with an atmospheric pressure ionization mass spectrometry, desorbing species upon heating of Si/Al<inf>2</inf>O<inf>3</inf> samples are identified: evident desorption peaks are observed around 400 °C for all spectra. The spectrum for m/e = 18, an indication of H<inf>2</inf>O, illustrates that gaseous desorption from Al<inf>2</inf>O<inf>3</inf> and from the Si substrate itself continues up to 600 °C and 700 °C, respectively. Also, it is shown that in the case of a 30 nm Al<inf>2</inf>O<inf>3</inf> layer, blistering starts at same annealing temperatures as gaseous desorption begins. In the case of a thin enough (< 10 nm) Al<inf>2</inf>O<inf>3</inf> film, blistering does not show. To complete the proof, elastic recoil detection measurements clearly show that after annealing a thick Al<inf>2</inf>O<inf>3</inf> film above 400 °C the H content is higher near the c-Si interface as compared to the near surface. Fortunately, effective lifetime and capacitance voltage measurements show that 5 to 10 nm Al<inf>2</inf>O<inf>3</inf> layers can still be adequate passivation layers after being annealed in N<inf>2</inf> environment at temperatures up to 500–700 °C: (i) interface trap densities (D<inf>it</inf>) can remain below 1×10¹¹ cm⁻² and (ii) fixed charge densities (Q<inf>f</inf>) stay negative and in the order of −3×10¹² cm⁻² Random local Al back surface field (BSF) solar cells, fabricated using a blistered film as rear surface passivation and no additional contact opening step, clearly show that random local BSFs are created upon firing of a blistered rear passivation layer covered by metal. Therefore, it is clear that blistering should be avoided, since it will reduce the overall rear surface passivation. The key to avoid blistering is using 5 to 10 nm Al<inf>2</inf>O<inf>3</inf> passivation layers and performing an annealing step prior to capping and co-firing. Al<inf>2</inf>O<inf>3</inf>/SiN<inf>x</inf> passivated local Al BSF p-type Si solar cells are made using an out-gassing step with temperatures up to 700 °C. For these cells, the reduction in blistering and hence improvement in rear surface passivation is clearly reflected in the gain in average Voc as a function of out-gassing temperature.

Low leakage high breakdown e-mode GaN DHFET on Si by selective removal of in-situ grown Si 3 N 4

We describe the fabrication and characteristics of high voltage enhancement mode SiN/AlGaN/GaN/AlGaN double heterostructure FET devices. The Si3N4 not only acts as a passivation layer but is crucial in the device concept as it acts as an electron donating layer (1). By selective removal under the gate of the in-situ SiN, we realize e-mode operation with a very narrow threshold voltage distribution with an average value of +475 mV and a standard deviation of only 15 mV. Compared to the reference depletion mode devices, we see no impact of the e-mode architecture on the breakdown behaviour. The devices maintain very low leakage currents even at drain biases up to 80% of the breakdown voltage.

Spatially-separated atomic layer deposition of Al 2 O 3 , a new option for high-throughput si solar cell passivation

A next generation material for Si surface passivation is atomic layer deposited (ALD) Al<inf>2</inf>O<inf>3</inf>. However, conventional time-resolved ALD is limited by its low deposition rate. Initially, a high-deposition-rate prototype ALD reactor based on the spatially-separated ALD principle has been developed, with Al<inf>2</inf>O<inf>3</inf> deposition rates up to 1.2 nm/s. Later, the spatial ALD technique has been transferred to an actual in-line process development tool (PDT) for commercial high-throughput ALD of Al<inf>2</inf>O<inf>3</inf>, resulting in a deposition rate of 30 nm/min. The passivation quality and uniformity of the spatially-separated ALD Al<inf>2</inf>O<inf>3</inf> films are evaluated on p- and n-type Si, applying quasi-steady-state photo-conductance, carrier density imaging and infrared lifetime mapping. In all cases, a spatial ALD Al<inf>2</inf>O<inf>3</inf> layer of only 10 nm reached an excellent passivation quality and uniformity, comparable to reference wafers passivated by equivalent temporal plasma-assisted or thermal ALD Al<inf>2</inf>O<inf>3</inf>. Effective surface recombination velocities as low as 1.1 or 2.9 cm/s were obtained after annealing at 350 °C or firing, respectively. Using spatial ALD Al<inf>2</inf>O<inf>3</inf> passivated local Al back surface field p-type Si solar cells, the sufficient passivation of this high-throughput Al<inf>2</inf>O<inf>3</inf> layer is evaluated: an average gain in open circuit voltage as compared to SiO<inf>x</inf> rear passivated i-PERC cells is obtained.

AlGaN Schottky Diodes for Detector Applications in the UV Wavelength Range

In this paper, the performance of AlGaN extreme-ultraviolet (EUV) detectors is optimized by a combination of experimental results, TCAD simulations, and theoretical modeling. Using the verified thin-surface-barrier model, key issues in technology development are identified. A first conclusion is that reducing surface defects at the metal-AlGaN interface is found to reduce diode leakage considerably, hence improving detector sensitivity. Evaluating the benefit of a fingered Schottky contact results in a second conclusion, as a semitransparent fully covering Schottky contact is found to provide a good compromise between EUV sensitivity and reduced leakage. Both conclusions are supported by experimental results.

Backside-Illuminated GaN-on-Si Schottky Photodiodes for UV Radiation Detection

In this letter, we report on the fabrication of near-ultraviolet photodetectors based on gallium nitride (GaN) layers grown on a Si(111) substrate. Optoelectronic characterization was performed using front-side and backside illumination, the latter possible by locally etching the Si substrate under the detectors using reactive ion etching. The dark current after removal of the Si substrate decreased by two orders of magnitude to around 20 fA at -1 V for a 300-mum-diameter Schottky photodiode. Responsivity at the cutoff wavelength (370 nm) was equal to 35 mA/W for the backside illumination. Detection at smaller wavelengths was not possible due to a nonoptimized layer stack. These first results do however illustrate the potential of backside-illuminated GaN-on-Si Schottky photodiodes in 2-D UV imagers.

Contactless electroreflectance evidence for reduction in the surface potential barrier in AlGaN/GaN heterostructures passivated by SiN layer

Contactless electroreflectance (CER) has been applied to study the AlGaN potential-barrier height in AlGaN/GaN heterostructures without and with a SiN passivation layer. In the case of an unpassivated structure, an AlGaN band-edge signal with a strong Franz–Keldysh oscillation (FKO) was observed. On the basis of the FKO period, the surface potential barrier has been determined to be ∼1.1 eV. For the SiN passivated structure, a broad CER signal without FKO appears at the AlGaN edge. This observation is associated with a decrease in the height of the surface potential barrier, i.e., a shift in the Fermi level position at the AlGaN surface toward the conduction band.

AlGaN photodetectors for applications in the extreme ultraviolet (EUV) wavelength range

We report on the fabrication of Schottky-diode-based Extreme UltraViolet (EUV) photodetectors. The devices were processed on Gallium Nitride (GaN) layers epitaxially grown on 4 inch Silicon (111) substrates by Metal-Organic Chemical Vapor Deposition (MOCVD). Cutoff wavelength was determined together with the spectral responsivity measurements in the Near UltraViolet (NUV) range (200nm to 400nm). Absolute spectral responsivity measurements were performed in the EUV range (5nm to 20nm) with the synchrotron radiation using the facilities of Physikalisch- Technische Bundesanstalt (PTB), located at Berliner Elektronenspeicherring-Gesellschaft fuer Synchrotronstrahlung (BESSY). The described work is done in the framework of the Blind to Optical Light Detectors (BOLD) project supported by the European Space Agency (ESA).

Illumination level independence in relation to emitter profiles on industrial high efficiency local Al-BSF cells

If the worlds answer to alternative energy production is to be silicon photovoltaics, the fabricated devices need to be robust and highly efficient in varying operating conditions. Rear oxide passivated local Al-BSF cells have a prevalent issue that hinders their performance in particular operating conditions, this issue being bias light dependence (reduced response at low illumination levels). It has been demonstrated by many [1,2,3] that to obtain high quantum efficiency at longer wavelengths, the cells need to be illuminated with a sufficient high level of bias light. If quantum efficiency is shown to be bias light dependant, at low light conditions the cell will simply underperform. Although most cells suffer from this type of degradation, in practice some high efficiency cells reach maximum spectral responsivity already at 0.3 suns and are considered to be bias independent. Regardless if in practice, cells can be named bias independent, the mechanisms for bias dependency is a very relevant characteristic of high efficiency solar cells of the present and future. In this paper we present a phenomenon that has been observed and repeated in separate experiments. We compare differences in rear passivation, specifically between a fresh deposited silicon oxide versus one that has been used also as a diffusion mask. We observe a relationship between bias dependence and the process flows as well as a relationship with the densification recipe. As expected the 90 ohm/sq emitter outperforms the higher doped emitters in the UV wavelength range. What is not expected is that when measuring without a bias light, the higher sheet resistance emitters outperform the lower sheet resistance emitters in wavelengths above 700nm. Another observation is that the cells that have been passivated with fresh silicon oxide indicate a large bias dependence at ultra low bias levels below .01 sun, but saturate performance above .05 sun. The diffused silicon oxide cells increase their quantum efficiency as the bias light is increased. The cells studied in this paper are fabricated using 148.25 cm2, 160μm p-type Cz-Si wafers with screen printed Ag front contacts and are rear-side passivated with a deposited rear silicon oxide/silicon nitride stack. The highest efficiency of the cells studied is 19.2 % with a Voc of 637mV, Jsc of 38.2 mA/cm2 and a fill factor of 79.1%

Experimental investigation of LF dispersion and IMD asymmetry within GaN based HEMT technology

An experimental investigation of the low-frequency dispersion affecting the behaviour of microwave devices is reported in this work. The study has been carried out by exploiting two different measurement techniques and experiments have been performed on a GaN based HEMT. In particular, bias and frequency dependence of dynamic characteristics has been clearly observed. Moreover, asymmetric behaviour not exclusively ascribed to the measurement environment (e.g., termination impedance networks) manifests in the non-linear response of the considered device.

A process technology toolbox for next generation large area crystalline silicon solar cells

For further reduction of the crystalline Silicon solar cell cost/Wp, a dual approach is required: Further reduction of the silicon material by using thinner wafer and further increasing the conversion efficiency. Considering wafer thicknesses of 150µm and below the standard process with Ag screen-printed contacts on 50–60Ω/sq emitter and full Al BSF cannot provide the necessary efficiency increase. The reason for that is the increasing influence of the rear surface recombination current, which becomes a limited current loss mechanism. Within the Photovoltaic department in IMEC a research program has been launched with the goal of providing industrial processes for the next generation thin crystalline silicon solar cells. In this paper we are reporting on the development of a process toolbox that allows overcoming the full Al-BSF and the Ag-screen printing front-side metallization limitations. The next step towards higher efficiency targets is the implementation of novel emitter schemes and consequently advanced front-side metallization like electro-plating of copper for further photocurrent and fillfactor increase. By implementing Cu-plating as a front-side metallization, large area cells with efficiencies up to 18.4% have been fabricated. These are the initial steps for a cell concept that potentially can reach 19% efficiency in an industrial process flow. The progress towards an industrial Passivated Emitter and Rear Locally doped cell concept (i-PERL) is presented.