Patent Description:
NPIN and NIPN modulators are known to have extraordinary benefits for high bandwidth. As known in the art, N stands for an n-type doping region, P stands for a p-type doping region, and I stands for an undoped intrinsic region. See, e.g., <NPL>; <NPL>; and <NPL>. However, these disclosures do not mention that NPIN and NIPN structures form a two-terminal heterojunction phototransistor with a photocurrent gain (beta) up to <NUM>. Such a device would amplify DC photocurrent to levels which make it wholly impractical in real systems. Power dependence of performance, modulation pattern dependence (low optical signal-to-noise ratio (OSNR)), poor reliability, and high-power dissipation are a few of the problems that could result. The only mention in the literature is a reference to being "cautious about current leakage" [<NPL>," cited above]. It was associated with sidewall damage during dry etching rather than a phototransistor effect. <CIT> describes a non-linear and bistable optical device. <CIT> describes a semiconductor device for controlling light using multiple quantum wells. <NPL>, XP011767410; ISSN: <NUM>-<NUM>, DOI: <NUM>/JLT. <NUM> describes "<NUM>-GHz Bandwidth and <NUM>. 5V V(pi) in <NPL> describes "<NUM>-Gb/s Low-Driving-Voltage InP DQPSK Modulator with an n-p-l structure". <CIT> describes microstructure enhanced absorption photosensitive devices. <CIT> describes a semiconductor device.

While NPIN and NIPN modulators have been shown as beneficial for high bandwidth, the phototransistor effect has not been identified. For practical uses of such modulators, these effects must be suppressed.

The present disclosure relates to systems and methods for suppression of phototransistor gain in an optical modulator fabricated from III-V semiconductors, having either an NPIN or NIPN structure. The present disclosure can utilize the NPIN structure such as described in Kikuchi, Nobuhiro, et al. (<NUM>) or in Y. Ogiso et al. The modulator core is in the intrinsic or undoped region. For an NPIN structure, the present disclosure includes a first upper clad with N-type doping having a first bandgap energy (Emitter), a second upper clad with P-type doping and having a second bandgap energy (Base), and a third upper clad disposed between the first upper clad and the second upper clad with N-type doping and having a third bandgap energy (Sub-Emitter). The third bandgap energy is less than both the first bandgap energy and the second bandgap energy. The present disclosure includes an extra layer, referred to as a sub-emitter layer. The use of a narrow bandgap n-type material between P and N layers is novel. Alternatively, the present disclosure can utilize the NIPN structure in Y. Ogiso et al. The approach described herein is a key enabler for high bandwidth optical modulators (<NUM> and beyond).

In an embodiment, an optical modulator includes an emitter layer with N-type doping having a first bandgap energy; a base layer with P-type doping having a second bandgap energy; and a sub-emitter layer disposed between the emitter layer and the base layer, wherein the sub-emitter layer has a third bandgap energy that is less than both the first bandgap energy and the second bandgap energy. The optical modulator can further include a collector layer having n-type doping; and an undoped layer between the collector layer and the base layer. The optical modulator can include an NPIN junction or an NIPN junction. The sub-emitter layer provides a barrier to electrons flowing from the emitter layer, while allowing photo-generated holes to recombine in the sub-emitter layer thereby mitigating current amplification. The optical modulator can further include a first contact disposed to the emitter layer; and a second contact disposed to the collector layer. The third bandgap energy can be about <NUM> to <NUM> Q where Q is bandgap measured in µm. The third bandgap energy can be about <NUM> Q where Q is bandgap measured in µm. The optical modulator is fabricated from III-V semiconductors. The sub-emitter layer can include a quaternary alloy that is either P-based (InGaAsP) or Al-based (InGaAIAs) which is lattice-matched to InP.

In another embodiment, a method of forming an optical modulator includes forming an emitter layer with N-type doping having a first bandgap energy; forming a base layer with P-type doping having a second bandgap energy; and forming a sub-emitter layer disposed between the emitter layer and the base layer, wherein the sub-emitter layer has a third bandgap energy that is less than both the first bandgap energy and the second bandgap energy. The method can further include forming a collector layer having n-type doping; and forming an undoped layer between the collector layer and the base layer. The optical modulator can include an NPIN junction or an NIPN junction. The sub-emitter layer provides a barrier to electrons flowing from the emitter layer, while allowing photo-generated holes to recombine in the sub-emitter layer thereby mitigating current amplification. The method can further include forming a first contact disposed to the emitter layer; and forming a second contact disposed to the collector layer. The third bandgap energy can be about <NUM> to <NUM> Q where Q is bandgap measured in µm. The third bandgap energy can be about <NUM> Q where Q is bandgap measured in µm. The optical modulator is fabricated from III-V semiconductors. The sub-emitter layer can include a quaternary alloy that is either P-based (InGaAsP) or Al-based (InGaAIAs) which is lattice-matched to InP.

Again, the present disclosure relates to systems and methods for suppression of phototransistor gain in an optical modulator fabricated from III-V semiconductors, having either an NPIN or NIPN structure. The present disclosure can utilize the NPIN structure such as described in Kikuchi, Nobuhiro, et al. (<NUM>) or in Y. Ogiso et al. The modulator core is in the intrinsic or undoped region. For an NPIN structure, the present disclosure includes a first upper clad with N-type doping having a first bandgap energy (Emitter), a second upper clad with P-type doping and having a second bandgap energy (Base), and a third upper clad disposed between the first upper clad and the second upper clad with N-type doping and having a third bandgap energy (Sub-Emitter). The third bandgap energy is less than both the first bandgap energy and the second bandgap energy. The present disclosure includes an extra layer, referred to as a sub-emitter layer. The use of a narrow bandgap n-type material between P and N layers is novel. Alternatively, the present disclosure can utilize the NIPN structure in Y. Ogiso et al. The approach described herein is a key enabler for high bandwidth optical modulators (<NUM> and beyond).

<FIG> is a diagram of a conventional indium phosphide (InP) PIN modulator <NUM>. The InP PIN modulator <NUM> includes a multi-quantum well (MQW) region <NUM> with doped upper/bottom (P-InP, N-lnP) claddings, layers <NUM>, <NUM>. This is an optical ridge waveguide in which light is phase modulated. A DC electric voltage <NUM> is applied across the p- and n-layers <NUM>, <NUM> keep the PIN stack-up in reverse bias. An AC voltage <NUM> dynamically modulates the phase of the optical signal.

<FIG> is a cross-sectional diagram of the PIN modulator <NUM>. The PIN modulator <NUM> includes top and bottom contacts <NUM>, that connect to the voltages <NUM>, <NUM>. There are the layers <NUM>, <NUM> and the MQW region <NUM>. Light propagates in the MQW region <NUM> from left to right. Bandwidth is limited by p-cladding contact/series resistance.

<FIG> is a cross-sectional diagram of a NPIN modulator <NUM> that provides improvements over the PIN modulator <NUM>. The NPIN modulator <NUM> includes the contacts <NUM>, the layer <NUM>, and the MQW region <NUM>, and further includes the layer <NUM> having an emitter n-InP layer <NUM> and a base p-InP layer <NUM>. Here, the p-doping in the layer <NUM> is replaced with more conductive n-doped material. The n-doped structures in the emitter n-InP layer <NUM> have reduced contact resistivity. The p-doped layer <NUM> is a current blocking layer. Such structures have the highest reported bandwidth in the literature.

Substitution of the PIN stack-up of the PIN modulator <NUM> with the NPIN in the NPIN modulator <NUM> improves device performance. The NPIN stack-up, however, resembles a typical NPN bipolar junction transistor (BJT), which could amplify the photo-generated current in the MQW region <NUM>. This photo-transistor effect is detrimental to the device operation and needs to be suppressed/eliminated.

<FIG> is a diagram of the NPIN modulator <NUM> for describing the phototransistor effect problem. With lights "on," the NPIN structure acts as a photocurrent amplifier; such devices have been commercialized as light detectors in other industries where amplification is desirable. For modulator design, large photocurrent will cause excessive power dissipation and loss of modulator efficiency; this phenomenon needs to be suppressed. Of note, there has not been a discussion in academic journals for high performance modulators of how to address this problem.

<FIG> is a diagram of an NPIN modulator <NUM> according to the present disclosure with a heterostructure for reducing the photo-transistor effect problem with the NPIN modulator <NUM>. <FIG> is a cross-sectional view of an epitaxy layer structure of the NPIN modulator <NUM>. The NPIN modulator <NUM> includes the contacts <NUM>, a collector n-InP layer <NUM>, the MQW region <NUM> (intrinsic region), a base p-InP layer <NUM>, a new blocking sub-emitter narrow n-type layer <NUM>, and an emitter n-InP layer <NUM>.

Of note, the description herein focuses on the NPIN modulator <NUM>. Those skilled in the art will recognize the blocking sub-emitter narrow n-type layer <NUM> can also be used with an NIPN modulator.

The emitter n-InP layer <NUM> has a first bandgap energy. The base p-InP layer <NUM> has a second bandgap energy. The sub-emitter narrow n-type layer <NUM> has a third bandgap energy that is less than both the first bandgap energy and the second bandgap energy.

The blocking sub-emitter narrow n-type layer <NUM> is a narrow-band structure to suppress amplification. It is not possible to make band-gap too narrow due to optical loss and the sweet spot for the blocking sub-emitter narrow n-type layer <NUM> is around <NUM> - <NUM>. 2Q, for C-band operation; note, Q is the bandgap measured in µm. <FIG> is a graph of transistor current amplification versus band-gap. Curves B, R respectively are associated with InGaAsP and InGaAIAs sub-emitter systems. It further demonstrates that only the bandgap of the sub-emitter is of essence not the particular alloy (P-based vs. Al-based).

<FIG> is an InP NPIN band diagram of reverse bias with and without light for the NPIN modulator <NUM>. <FIG> is an InP NPIN band diagram of reverse bias with and without light for the NPIN modulator <NUM> with the blocking sub-emitter.

<FIG> is a graph of photo-transistor current gain versus sub-emitter bandgap in the NPIN modulator <NUM>. At an intersection <NUM>, marked "w/o blocking sub-emitter" and "InP bandgap", respectively, is the gain (<NUM>) of the modulator <NUM> if no measures are taken to suppress phototransistor action. A line <NUM> labelled "w/ wide bandgap base" is the effect of the Prior Art of incorporating an InAlAs "blocking layer" into the base. The current gain is <NUM>. A line <NUM> below at any energy gap < <NUM> eV is the beneficial effect of this disclosure. The gain is reduced to the theoretical low limit of <NUM>. Physically, the sub-emitter provides a barrier to electrons flowing from the emitter, while allowing photo-generated holes to recombine in the sub-emitter layer thereby mitigating current amplification.

This section includes a set of electrical simulations showing that NPIN structures are capable of transistor action (current amplification). Simulations were initial exercises to understand the extent of the problem and serves as a brief review of transistor action in a vertical NPN BJT.

<FIG> is a circuit diagram of the NPIN modulator <NUM>. The NPIN modulator <NUM> is a BJT. <FIG> is a graph of doping profile of the NPIN modulator <NUM>. <FIG> is a graph of the electric field in the NPIN modulator <NUM>. <FIG> is a graph of the conduction band energy as a function of applied external bias of the NPIN modulator <NUM>.

<FIG> are graphs of current versus voltage when the BJT is turned on. Of note, the current (I) and voltage (V) are shown for the base (b), emitter (e), and collector (c).

<FIG> is a graph of DC Current Gain (β) for the BJT. <FIG> is a graph of Current Transfer Ratio (α) for the BJT.

This section simulates the baseline performance of a conventional PIN diode structure with optical stimulus, i.e., the PIN modulator <NUM>. This establishes the baseline photocurrent levels for these models and sets expectations for what needs to be achieved from any new NPIN designs.

<FIG> is a diagram of a physical layout of the PIN modulator <NUM> in the y and x dimensions, for reference and use with <FIG>.

<FIG> are graphs of the PIN photo response when short-circuited/reverse biased. <FIG> is a graph of conduction band, <FIG> is a graph of electric field, <FIG> is a graph of current-light characteristics, and <FIG> is a graph of DC IV characteristic. Photocurrent varies linearly with input optical power. A photocurrent of ~<NUM>. 7µA flows with no applied bias, i.e., short-circuit current. Photocurrent is independent of applied voltage.

<FIG> are graphs of the PIN photo response with open-circuit voltage. <FIG> is a graph of conduction band, <FIG> is a graph of electric field, <FIG> is a graph of electron concentration, and <FIG> is a graph of hole concentration. Accumulation of excess electrons (holes) on the left (right) hand side of the intrinsic layer creates a dipole whose electric field opposes and cancels the built-in field of the p-i-n structure. Therefore, in the middle of the intrinsic region the drift (E=<NUM>) or diffusion (n,p are uniform) components of the current vanish for both type of carriers. The open-circuit voltage of the structure is equal to <NUM>.

<FIG> is a circuit diagram of the open-circuit voltage for the PIN photo response. The open-circuit voltage is, in essence, equivalent to an optically induced bias.

This section explores the performance of the NPIN modulator <NUM> that does not use the proposed heterostructure. Note that the nominal photocurrent levels are nearly 100x larger than what was achieved for a conventional PIN structure. A subsequent series of simulations demonstrates that the current amplification factor cannot be easily suppressed by changes to the geometry of the structure Although not described herein, the design changes considered are impractical as they would impose significant penalties to the device bandwidth.

<FIG> is a diagram of a physical layout of the NPIN modulator <NUM> in the y and x dimensions, for reference and use with <FIG>.

<FIG> are graphs of a baseline photo response for the NPIN modulator <NUM> in. <FIG> is a graph of DC IV characteristic, <FIG> is a graph of the electric field, and <FIG> is a graph of the conduction band. It takes only ~10meV to forward bias BE diode. At ~300meV, photocurrent saturates at βmax=<NUM>.

<FIG> are graphs of the effects of reducing emitter doping. <FIG> is a graph of DC IV characteristic, <FIG> is a graph of the electric field, and <FIG> is a graph of the conduction band. By reducing the emitter doping to 1E17, βmax will decrease to ~<NUM>. Moreover, field distribution in the intrinsic region resembles closer to that of an ideal PIN.

<FIG> are graphs of the effects of widening the base. <FIG> is a graph of DC IV characteristic, <FIG> is a graph of the electric field, and <FIG> is a graph of the conduction band. βmax may be reduced to ~<NUM> by widening the base (<NUM>).

<FIG> is a graph of variation of β versus Emitter Doping and Base Width.

This section demonstrates the relationship between band-gap of the proposed emitter layer and photocurrent amplification. The primary conclusions include.

<FIG> is various graphs for InGaAs (narrow bandgap), InP, and InAIAs (wide bandgap) illustrating the conduction band and the valence band.

<FIG> is a diagram of a physical layout of the NPIN modulator <NUM> in they and x dimensions, for reference and use with <FIG>.

<FIG> are graphs of the InGaAs-NPIN Photo response (Narrow BG Emitter). <FIG> is a graph of DC IV characteristic, <FIG> is a graph of the electric field, and <FIG> is a graph of the conduction band. βmax is curbed at <NUM>. E-field profile is not compromised in the MQW region. A narrower bandgap at emitter (relative to base) suppresses β.

<FIG> are graphs of the InAIAs-NPIN Photo response (Wide BG Emitter). <FIG> is a graph of DC IV characteristic, <FIG> is a graph of the electric field, and <FIG> is a graph of the conduction band. βmax is enhanced to <NUM>. E-field profile in the MQW region is significantly compromised. A wider bandgap at emitter (relative to base) enhances β.

<FIG> is graphs of the heterostructure NPIN photo response. If x=<NUM>-y, then InGaAIAs is lattice matched to InP.

<FIG> are graphs of gain vs. blocking layer thickness and composition. <FIG> is a bandgap graph, <FIG> is a graph of β vs. blocker position, and <FIG> is a graph of β vs. blocking layer thickness. <FIG> is a graph of gain vs. blocking position.

<FIG> is a graph of electric field for InGaAIAs vs. InGaAsP as blocking layers. <FIG> is a graph of Q for InGaAIAs vs. InGaAsP as blocking layers.

For photocurrent calibration, Narrow bandgap (BG) @ Base leads to β enhanced, Narrow BG @ Emitter leads to β suppressed, and Wide BG @ Base leads to β being suppressed, but not as much. Due to fabrication and material constraints, this latter approach is not as effective in suppressing unwanted current as the solution described herein.

<FIG> are graphs of the InAlAs-NPIN Photo response (Wide BG Base). <FIG> is a graph of DC IV characteristic, <FIG> is a graph of the electric field, and <FIG> is a graph of the conduction band. βmax is reduced to <NUM>. E-field profile is slightly weakened in the MQW region. A narrower bandgap at emitter (relative to base) suppresses β.

A simulation to assess device sensitivity to temperature; product operates at <NUM> while most simulations were conducted at <NUM>. This depicts photocurrent amplification as a function of emitter band-gap at three temperatures. The suppression mechanism is robust up to <NUM> for materials with bandgaps in the <NUM> to <NUM> range.

<FIG> is a graph of current amplification vs sub-emitter band gap for three different temperatures. <FIG> represents the same thing as <FIG>, but the bandgap is measured in terms of its wavelength (Q) instead of eV.

The preceding sections focus on vertical current flow of a NPIN structure; fabricated devices are finite and need to be terminated. This section depicts expectations for the edge of fabricated devices and considers whether non-idealities will degrade or destroy the advantage of the proposed approach. The models predict that performance is insensitive to anticipated issues. This conclusion is drawn from a simulation of a conventional PIN structure as a baseline for subsequent results and a simulation of proposed NPIN structure while varying the geometry of the proposed narrow band-gap layer.

<FIG> is a perspective diagram of the edge region of a fabrication NPIN structure. <FIG> is a cross sectional diagram of the edge region of a fabrication NPIN structure. There are two NPIN devices: (i) The primary vertical structure, and (ii) a parasitic structure at the edge of the device. The narrow band-gap material may not cover the entire sidewall leading to a question of whether there would be elevated leakage.

<FIG> is graphs of a simulation of a conventional 2D PIN Edge. <FIG> is graphs of a NPIN with <NUM>. 2Q -with a complete sidewall. <FIG> is graphs of a NPIN with <NUM>. 2Q with the sidewall removed. <FIG> is graphs of NPIN current as <NUM>. 2Q sidewall is pulled back.

<FIG> is a graph of sensitivity showing current increase with temperature; for variable pullback. <FIG> is a graph of sensitivity showing Current vs. affinity offset; variable pullback; at <NUM>.

Claim 1:
An optical modulator (<NUM>) comprising:
an emitter layer (<NUM>) with N-type doping having a first bandgap energy;
a base layer (<NUM>) with P-type doping having a second bandgap energy; and
a sub-emitter layer (<NUM>) disposed between the emitter layer (<NUM>) and the base layer (<NUM>), wherein the sub-emitter layer (<NUM>) has a third bandgap energy that is less than both the first bandgap energy and the second bandgap energy;
a collector layer (<NUM>) having N-type doping; and
an undoped layer (<NUM>) between the collector layer (<NUM>) and the base layer (<NUM>), wherein the undoped layer is configured to have light propagate through the undoped region for modulation thereof.